Yes! I was thinking something very similar. Though, I didn't mention it because the objection had been raised that having two lists seemed complicated, and that would certainly complicate it.
I would also point out that that would prevent people from taking the same list for both (unless they wanted to keep to troops & Elites).
Similar alternative ideas:
* Attacker must take 1 FA, Defender 1 Heavy.
*All lists are limited to 1 slot of Elite, FA and Heavy, (we are only talking 1000 points) but Attackeers can get full FA, Defenders full Heavy
* Maybe FA is scoring (and denial, this case) for attackers, Heavies are scoring for Defenders. I might like this best as I think generally Heavies are seen as more powerful, but FA are of course faster, so scoring seems a little more useful for them.
Completely separate but let's do it because I say it's awesome idea -- DA get to run ravenwing as attackers and deathwing as defenders without taking the requisite special characters.
I assume people have to take the same codex for both lists?