Would suggest the 1+1 for a number of reasons.
1) Some may prefer ArmA1 (some of us still like DoD 1.6 over DoD:S) though IAH after seeing watching some videos I can't see why.)
2) Some may not be able to run
ArmA 2 right away. (ie: I'm not building a new rig until the fall. Summers are too short)
3) Some may sit on the fence to see how
ArmA 2 evolves.
The 2 servers wouldn't compete against each other but would compliment each other and
TG by offering a place to play to a wider audience. You may even find your ArmA1 server increasing in popularity as so many single server groups switch over to ArmA2 leaving those playing ArmA1 scrambling for a place to play.
Also offers some stability in that those regulars that don't/can't run ArmA2 still can play on a
TG server.