Neo, Tom's correct that there will be an imbalance.
One thing to make clear: agile is a framework for having developers figure out how to deal with this rather than relying on a defined process to handle it for them, so there is no "best practices answer" to your issue.
Since this issue comes up a lot, I'll share what some folks have done:
- Less strict specialization: The artists tests and tunes what is available if no art work is needed.
- The programmer iterates more frequently. E.g. instead of building an entire camera management system and a set of cameras, they apply the YAGNI principle and just provide a simple polar, follow or 1st person camera to begin with and some tuning interface. Again, with the artist doing some tuning work, it can allow more iteration on the game, which is good.
- I have even seen times when the artist picks up a little scripting work (depends on the artist).
- Take advantage of the extra art time. When we were building a mechanic and a city to use it in, we had the artist build a grey-box full-sized city as well as representative city blocks with higher fidelity. That allowed us to approach the mechanic's gameplay from both sides of the fidelity as well as giving the programmer a range of assets to test with.
- Have the artist detail your car in their spare time (just kidding, but this was suggested once ;)
Iterative development not only applies to what you're making, but how you're making it as well. Experiment & have fun!