Okay SPOILER ALERT.

I know the ending of MGSV that Venom Snake is not the real Big Boss. So now my question is that was the real Big Boss ever in a coma ? If so how the hell did they both wake up at the same time ? If Big Boss was not in a coma then why was Venom Snake in a coma ?

Or was the coma of Venom Snake induced so that Big Boss can fully recover ? If so how was that piece of metal lodged in his skull ? 

Need help, a lot of confusion. Since I am a Metal Gear noob.