Allow me to try to talk you out of that idea. The plane was designed with 4 aileron servos for a reason: At this model size 16lbs plus, you really need the most solid control set-up possible - no shortcuts. The stock ailerons don't have hardpoints in them at the trailing edges (where you'd normally locate the inter-airleron horns), so you'd be hoping the balsa is up to the job. I would strongly discourage this practice in a giant scale plane.

If you want to save some bucks, you can run cheaper ballbearing servos like the Hitec 635, but pls use 4 of them! It would be better even to go w/ 4 Futaba 3001 standard servos running on 6 volts, than to slave the top ailerons to the bottom ones.