One technical founder will not be able to do as much work as a 2 technical founders. So the downside is less man hours to get your product coded and polished. This is assuming everyone is equally talented. I think it's very inspiring to see two motivated and talented people together set on one goal with nothing else in mind. Also, with 2 people, it's less likely to get depressed because you are always trying to appear motivated for the other person. You just don't want someone unmotivated or without a high value add. He or she should add as much value as you.