If the two servos you are using for the elevators have different speeds or reaction time you can not fix this with the programming. I've seen this even with same model servos. However you can set each servo subtrim and endpoints independently as long as they are on separate channels. This is the way all giant scale pilots do it. You can use whatever channel you have but 8 is common and some txs have a preprogrammed ailevator program which mixes 8 to 2 automatically. Don't use the aileron mixing on this feature as that is really for jet use. set the subtrims and endpoints of each channel independently and you should be good to go.