The reason Perry and Cain are not double-teaming Romney is because it wouldn’t do any good. The 20% or so of Republicans that support Romney already know he passed Romneycare, believes in AGW, etc. They support him anyways, and harping about these conservative issues they don’t care about will not erode Romney’s base of support.
Perry and Cain both need to knock the other one out of the race if they want to wrap up the conservative wing of the party. THEN, they will be able to go toe to toe with Romney without having another strong conservative candidate sniping at them from the sidelines.
Romney is doing a good job of knocking Perry out and greatly helping Cain in the process.
Undecided voters and those supporting the weaker candidates will break heavily towards Cain.