Can a Jedi apprentice be a Darkside apprentice at same time?

The short answer to the question is YES.

And now for the long answer:

1. Remember ObiWan (Ben) Kenobi's words to Luke when he gave Luke his father's lightsaber: "A young Jedi named Darth Vader who was a pupil of mine until he turned to evil, helped the Empire hunt down and destroy the Jedi knights. Vader was seduced by the Darkside." ObiWan's own words seem to indicate that Vader was still his pupil in the Lightside while he became seduced by the Darkside.

2. Palpatine knew of Anakin's dreams about Padmé dying. That suggests that Palpatine may have been more than just watching Anakin's dreams but they may have been a Darkside manipulation by Palpatine himself.

3. In ROTS ObiWan tells Padmé that "shortly after the death of Count Dooku, Anakin became (Palpatine's) new apprentice." Anakin's actual apprenticeship under Palpatine might have begun when Palpatine seductively urged him to kill Dooku, not when Anakin pledged himself to Sidious teachings. But up until Anakin pledged himself to Palpatine, ObiWan was still his master in the Lightside as numerous other lines in ROTS indicated.

4. In CWAS (Clone Wars animated series) season 2, there is a 2 part/2 episode story entitled "Twilight of the Apprentice". A Jedi padawan named Ezra Bridger meets (Darth) Maul on a planet forbidden to Jedi called Malachor. Ezra is unaware that Maul used to be a Sith Lord but is drawn into sympathy for Maul as they get to know each other through Maul's cryptic answers about who he is. They approach a door to a Sith temple that can only be opened by 2 Force users. Ezra learns under his Jedi master named Kanan. Before opening the door together Maul tells Ezra "Two must lift these stones. No more, no less. That is the way of the Sith." But Ezra is so focused on entering the temple to gain a holochron that allegedly contains knowledge to defeat the Sith, he helps Maul open the stone doors. It is at this very point he starts being Maul's apprentice unknowingly. Later on the story when Ezra and Maul became separated and Maul teams up with Ezra's Jedi master to fight a common enemy, Maul calls Ezra his apprentice to Kanan's surprise just before Maul turns on him and attacks him. Subsequent episodes show how Ezra attempts to follow both paths for a time.

5. When Rey calls on the Force in her duel with Kylo she eventually overpowered him and subdued him. It seems Rey was actually channeling the Darkside as she stood over Kylo contemplating on whether or not to kill him. This despite the fact that her connection to Force has been the Lightside calling to her through Luke's lightsaber and Force visions. In the novelization of TFA Rey actually hears a Darkside voice in her head telling her to kill Kylo.

So yes, you can be both, for a while, but you probably will get a result you don't want if you stick with it!  But of course that's a big message in the saga.  I don't know why Kenobi isn't quoted more:


And this is pretty interesting that Ben said that too, in that no Jedi in the prequels dared confront and go beyond their dark side.  Their way was to hide from it, avoid family, shun attachments, to reduce the chance of being tempted towards darkness.   So how did Ben get to this point?    What did Qui-Gon teach Yoda and Ben about the Wisdom of the Whills that the Jedi had lost?  

I think the path isn't to train in both the dark and the light, but to stay in the light while recognizing and owning your own darkness.

I suppose that's true. In references that I've seen to Mace Windu using the Vaapad it commented he "skirts dangerously close to the Darkside without going over". Whether that means that he was channeling both the Lightside and the Darkside in some controlled manner, or he was in a gray Force area that allowed him to get close to the Darkside without going over, I'm not sure.

But I was addressing more of the process of Darkside apprenticeship. Unlike the Lightside, a person can unknowingly begin apprenticing under the Darkside at the same time that they are learning under the Lightside. Under the right circumstances the seduction may take them to a point of no return without them fully realizing it. It's the difference in the seductive nature of Darkside apprenticeship as opposed to the Jedi or the Light.

When his own darkness did him in?  He had every opportunity to take the Emperor in with Anakin on his side, but he chose to do the wrong thing, (as the Emperor knew he would!) and the cost was the fall of the Republic.

Are you a parent?  When your kids do something wrong, is it because the school failed them?  The teacher failed them?  The stuff they watched on tv that day failed them?

This is the kind of thinking I reject to its ugly, pandering, evil, false core.  The story of Star Wars (based on the video you yourself posted) and the story of life in general is that we each control our own destinies and no God or Devil can be blamed for which choices we make.

Mace didn't fail himself.  Anakin failed.  Anakin is 100% to blame for everything going wrong.  It wasn't an outdated, patriarchal system that was to blame.  It wasn't circumstantial, nor was it complicated.

It was very simple.  Anakin had a choice to make.  A choice that he was warned about numerous times previous to his decision by the people who really cared about him, including Mace himself. 

Your assertion that Mace had the opportunity to take Palpatine in with Anakin's help is false.  The main motivating factor in Anakin's poor choice was his selfishness in wanting to save Padme.  It wasn't his distrust of the Jedi Order, nor the ways of the Republic, nor Mace's intent to sanction Palpatine directly.  Those motivations combined did not contain one-tenth the potency of his selfish desire to use Palpatine's knowledge to save Padme.  You know that, I know that and everyone who watched the movie knows that.

No matter what Mace had done in that scene, Anakin was going to betray the Jedi because betraying the Jedi was the only way he could save Padme.

The biggest difference in the way we see the world is that I don't think what Mace was about to do was either murder, nor was it wrong.  Evil must be sanctioned and it is always up to good people to do that.

Anakin should've done what his superiors told him to do and sit at home.  He was too immature and reckless to be involved in that situation.  The reason it went south in the first place is because he didn't obey orders.

Palpatine just murdered three Jedi and would've murdered Mace if playing the victim hadn't suited his cause.  From Mace's perspective, he had every right to lop Palpatine's head off.  It would have been self-defense, it would have been legal and it would have been just.

Morals in stories are no different than morals in real life.  You probably think the best way to deal with radical Islamic terrorism is to "talk it out with them," right?  Or, per you per Rose, we should allow terrorists to detonate their bombs so long as we go around providing medical aid to the victims so they survive.  Right?

He wasn't. Neither would I be if someone were trying to kill me and I instead killed them. I'd be guilty of no crime. And I don't even have half the leg to stand on that Mace did. He actually had the authority to make arrests in the first place.

The only thing the scene you quoted proves is that the other Jedi should've listened to Mace sooner and taken action when they had the chance.  Sometimes "taking great care" isn't the right maneuver.  When facing ultimate evil, quick, decisive sanctioning is the only way to go. 

Toward the end of WW2, America had a choice to make.  We could've "taken great care" and let things play out according to conventional military rules of the era.  This would have prolonged the war by years and extended the death toll on both sides.  Instead, America took quick, decisive action and dropped two nukes.  America will always be held accountable for that and in some debate circles will be the ones remembered in infamy. Sometimes there is no pure, benevolent, non-violent way through a crisis.  Sometimes heroes carry scars, even shame.  C'est la vie.  I'm glad we dropped those bombs, Mace and the other Jedi should've whacked Palpatine if/when they had a chance and the people in this world who criticize the methods required of life's heroes have the luxury of doing so, even though it signals their cowardice.