I have two trains of thought about 'doubles' in TII, O2 etc, both of which are working from the assumption that this is all MJ's huge 'reality movie'.
Firstly, he could be just the director of this 'movie' and therefore used doubles as actors to play him.
Or secondly, MJ is director AND actor, and it's all him, albeit sometimes 'altered' with make-up and his not insignificant acting skills.
(I suppose there could be a third option that it's a mix of the two!)
All I can say is I hope we get to find out for sure - I don't want this 'reality' thing to end and never be explained. I really hope it's all been documented and all our questions get answered in the fullness of time, like the credits at the end of 'Ghosts' (which I have to admit had me floored first time I saw it - I had no idea MJ was the mayor!!)