Spoiler alert for Mission 46 The Man Who Sold The World.

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 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?

In addition, what was that piece of metal lodged in his skull ?