Personally I had a big pain in ass by balancing between these two apects in development of our games. First thing you should have is a concept, then it goes to design document and specification. Then go sketches, to determine how the things will look like. Believe me, if you don't have sketches approved before you start the development - you're screwed. Like I did.
On early stage programmers can work without any art. Code don't require it. And when the code is done, they will need art to setup the game.
So in your plan take it into account, that the art should be done before programmer will finish his coding part. Because there will be gaps that you can't fill out during the process.
Balance is art. You'll never learn it until you do couple of games with a team.