I believe that BitDefender 8 freeware is not an resident scanner, just on demand scanner... it means it doesn't run in the background, so it won'r interfere with avast! in any way.
On the other hand, if you are talking about some version that has resident protection, advice will always be - DO NOT INSTALL two or more resident antiviruses on the same machine. You'll have nothing but headaches. Same situation like with firewalls...
Regarding BitDefender and disabling something inside so it wouldn't run on Windows sturtup, you'll have to search your answer in BitDefender forum, because we mainly deal with avast! in this forum. I don't even know if there is any BitDefender forum...
Anyway, with avast! you are more than protected... sometimes check your computer with some online scanner like Housecall and you'll be fine. We all do that, because we all like second opinion sometimes. BitDefender 8 Freeware is nice addition and without resident scanner it runs perfectly along with avast!.
Again, if you have some version with resident shield, my answer to your question is above.
