I had three jobs at once for awhile. I don't think all three together added up to full time. Right now I have two, both also part time even added together.

My friend worked 70-ish hour weeks at two jobs for the better part of a year. He was dead to the world. One of our games in Skype was everyone else trying to make him forget what day it was. It wasn't a very hard game. But he paid off all his student loans, got promoted at one job and quit the other, and is getting his own place soon.

So, yes, you will go insane. But if at least one of the jobs is low-energy (one of my friend's was parking garage attendant) then you won't die.