While you'll probably ignore the advice of everyone here telling you no, Just let me say.
NO, NO, NO, NO, NO, NO!!!
Do you want to stay friends with your roommate? Then don't do it. This will only serve to completely destroy your friendship.
Do you want to keep trusting your roommate? Then don't do it. This will only serve to completely destroy your trust in your roommate.
I wish I could instill into your heart and mind how terrible of an idea this is. Money is the #1 cause of divorce in America, you aren't even married to your roommate and you want to share the number 1 reason for destroyed relationships?
What happens when one of you over draws? Who's going to pay the bill? What happens when one of you gets a speeding ticket? Problems abound. And what do you gain? "Oh boy, we have our money in one spot!"