I've flown several 50cc class birds and find that min of 200oz on rudder is needed to hold a KE. I'm in process of building a QQ 102" Yak and plan to gang up 2 hitec 7955 for that massive rudder for KE loop authority. I talked to Quique about your question, and he personally likes to use one big torque servo like the JR 8711 that puts out 400oz rather than gang up 2 smaller torque servos and have to match them. My feeling is if you already have 2 lesser servos to use on the rudder, then gang them up. But if you have to buy new, then get that JR8711 and you'll be set. For elevators, you'll also need about 250oz each side. That's my 2 cents.