Having a co-founder lets you bounce ideas off one another. It lets everything you do take on its own natural evolution. It's hard to explain unless you have a co-founder that you can easily and quickly communicate with, but I'll try.
You could compare it the show House MD. House is a brilliant medical mind, and much better than anyone on his team. Why does he even have a team, you ask? To throw ideas back and forth. When he can't access his team, he'll go to anyone to keep the idea in the air. He's even used a bum and an ex-con.
As an individual, you can train in all the skillsets in the world. You can become better at front-end, back-end, business development, selling, talking to people... everything. You can be even be better at coming up with ideas.
But you'll never be better at developing ideas.
That's the strength of multiple founders.