As Mr67stang said, you could get an extra sevro but I really wouldn't worry about it. Especially if the trainer isn't set up for one. The reasoning for this is because you are going to have to tear into the wing and make modifcations in order to mount the two aileron servos in the wing. This isn't necessarily difficult if you have the experience, but it's something that people just starting out in the hobby don't have. My recommendation is to just fly the plane as the manufacturer set up. I've flown lots of 60 sized planes with one aileron servo and had no issues. All of my Ultra Sport 60's were flown with one aileron servo and it was more than enough for that plane.

Ken