I am in the exact same situation as you. 12 team league, 2 QBs, and my lineup is:
Plummer, Farve, A Smith (used to be Volek until I grabbed Smith)
The way that Plummer and Farve played last week has me worried. But I agree with the second poster, we only have 2 options:
1) Wait it out
2) Make a trade
The third option is to wait for someone to go down and then scour the waiver, but I'm guessing that will only work later in the season when there are more inuries.
For waiting it out, I do think that Farve is going to end up putting up decent numbers but with a lot of picks. The key with him is figuring out when to start/sit him. If I were you I would feel pretty good about Rivers as a 3rd- he looked very sharp on Monday, and although he didn't throw much, he's pretty much guaranteed to make TD passes with very few INTs the way they run that offense.
Brooks I would worry about. He got eaten alive out there. You might be better off with Farve/Rivers. In any case, I would start Rivers every week.
My strategy is to wait it out for another week, and if I think it's time to panic, work out a trade where I can buy low sell high. I might ship off one of my backup WR's (L Evans, R Smith, A Bryant, T Williamson, so I have a lot of depth here) who has a big game next week plus Marion Barber III to the Julius Jones owner or something like that.
But unless QB play improves or you work out a trade, we're screwed in a 2QB league, even if the depth at your other positions is exceptional. I won this league last year solely at the QB and WR spots because of great draft picks. I think QB play is huge in 2QB leagues...