Hah. Even aaa game companies rarely create their own engine. But the engines they use cost real moneys.
Deving together is like making a movie with several directors.
Can be done, but is hard to find directors willing to share the work . Better find dedicated programmer or artist. But professionals usually like to get paid. And non professionals, well, look at the other comments. Good advice there. Or just look at a discussion and changelog of some wikipedia pages. On the surface the strangers work well together, but under the hood, the flame wars and bickering happens ;-)