many avs's don't require you only use theirs - just read their rules. but you won't get TONS of signups from the traffic avs's send you as most of their traffic are already members. you will get some upsells from their traffic and a few signups. most of my money comes from selling members to my sites.

you won't want to have two avs's on one tour as that will make it twice as likely your potential members will already be members of one of the avs's. so mirror your tours if you want to use two avs's. you may be able to drive search engine traffic to both tours on both avs's. you can also make multiple tours - not mirrors, but several tours leading to one join page - and get search engine traffic to your avs sites that way.