My 2v2 Vetoes:
Every Composition:
Hurtgen Forest since it is the most terribly balanced map I have ever seen. Southern side has easier access to both fuel points and can defend them far better.
I.) As Axis:
Rzhev and Moscow Summer and/or Winter. Rzhev is too big for a 2v2 map in my opinion and heavily favours southern side. Moscow Summer/Winter allows to flank Allies from every possible side which makes it nearly impossible to setup your defensive units to defend fuel or cut off and leads mostly to disatvantage in early game. Furthermore is the southern side favoured in this map. Northern side suffers due to many buildings surrounding the cut off to your natural fuel point combined with the fact that building clearing measures takes too much time at this stage of the game. Allies can compensate spawning on northern side due to their high mobility in early game whilst Axis can be easily cornered due to their lameness in early game. OKW can probably be better in this case since they can set up forward heal head quarters. Consquently, they do not have to retreat to main base and need a shorter way back to cut off or fuel point.
II.) As Allies:
I think here you have to differentiate between double Soviets and mixed Allies/double USF.
1.) Double Soviets:
This combination allows you to adapt to any given situation, may it be bunkerous double OKW, versatile mixed Axis or flexible double Ostheer. You have great access to indirect fire (120mm mortar, Katjusha, howitzers), strong infantry/anti infantry (conspam, guards, shocks, sniper) and tanks (T34/85, IS-2, ISU-152, M4C Sherman) as well as powerful, formation disbanding ablilities such as the flame barrage as well as abilities which are suited to destroy Axis armour such as Mark Target. Due to all these strengths it isn't necessarily required to veto maps but to make life easier. I veto Ettelbrück Station, Rails and Metals and Minsk if I feel to since all these maps favour Axis (Volks-)Grenspam as well as MG42/34 and Axis armour.
2.) Mixed Allies/Double USF:
Having one Soviet player less or none at all does considerably shrink your options. USF has weaker indirect fire options and lacks strong allrounder tanks in late game. Their infantry doesn't scale very well to Axis counterparts in late game unless you upgrade them which costs munitions you need for abilities. USF has no real counter to it like snipers. So the main part of their army, armour, can be engaged by PaK40s/PaK43s and schreck blobs as well as Axis armour quite comfortably. Losing in the early game is therefore not an option for USF unless you want to fight a strenuous yet unforgiving uphill battle. Therefore, in my opinion USF has to play the armour counter role in mixed Allies team to make it possible for Soviet player to concentrate on anti infantry measures which are desperately needed if you consider blobbing. The best doctrine to choose for this quest is Airforce whilst Soviet player has to go for 120mm mortar doctrines.
Regarding double USF I cannot further elaborate because I lack experience of it but I can say that maps such as Rails and Metals are hell to play on for double USF since their flanking potential is restricted due to map design. Furthermore, Elephants or Jagdtigers limit armoured movements and combat possibilities by an huge amount.
Vetos for Mixed Allies: Ettelbrück Station, Rails and Metals, Minsk Pocket
Vetos for double USF: same like Mixed Allies