Well, after some testing, I have come to the following conclusion, try to stay with me here...
When doing a MvM, teams change around, team 1 as you know stays team 1, and team 2 as you know moves to team 3, now that works out fine and dandy, everyone gets that.
When doing a AVA, teams also change around, team 1 (being a marines based team) changes to team 2, and team 2 changes to team 4 (as I'm guessing team 3 is a marines based team also) now here are the things, since team 1 takes teams 2 place, the following thing happens:
team 's 1 main hive becomes teams 2's main hive, but team 4 wont be able ot hurt it as it's their hive
Team's 2 spawning works fine, it moves to four, but since team 2 is already taken, team 1 has no spawnning abilities, at all.
We have a map co_gar2v2 that sort of works, but it has many bugs,
one being that for some reason abilities will and will not work for some people (xeno, devour, might might not work) I have no clue why it does that,
two, Team one does not have any spawns... at all, so you spawn wherever your spectating, i figured a way to atleast get you to spawn inside the map, (out of RR) but if the team is full you'll be a "rr" unit (as we will just simply call them) in side the map.
There are ways to work around it, and the hives are all screwed up, so im not gonig ot bother release any map for it, if someone wants to make a buggy AVA, just to enjoy the onos devouring onos :-D (always fun) you can go ahead and pm me and Ill try to send what I can to help you with.
Sorry for the bad news, I know a lot of you were looking forward to AVA, hell i was too but from my point of view it's not possible, It still might be, but if it is I cant see the way (im not saying its impossible, because hey people said MVM was impossible
)
So if anyone wants to try and make a working one anyway gl (my theory is it works a different way then mvm dunno how though)