I know that many of the guys already signed up play together a lot, so I'm sure you have your reasons, but as an outsider eager to sign-up, this is my two-cents worth:
I agree with Hedgie. Forcing one player to choose DA, TB or Reg essentially forces the other player to choose Oak, LE or UB. And since your teammate has to play an assassin, you pretty much have to play Oak or Erebus. The team that picks first can't risk not playing Oak or LE, in case the other team does, and since they have, then the second team has to take one to counter. It pretty much eliminates UB, Rook, Sed, QoT and Occ from the tourney.
A simpler solution would be only one Tier 1 DG. Or heck, no Oak, LE or UB, if you really want to force people out of their comfort zones and see some variety. But I think the current format will only see a few viable teams: Oak/ TB, LE/ TB, Oak/ Reg and LE/ Reg. (DA players will scream about this, but in 2 on 2 he is much less ganktacular and really loses a lot of his effectiveness.) I would personally place my money on the winning team being Oak/ TB or LE/ TB, as those have the most lane control capability and entire-game effectiveness.
All that said, Chiefy01 and I will play regardless of the rules, cause any Demigod is good Demigod.