Ideally, you put each side on a different channel and mix them. This way you can adjust each side individually. Some of the newer radios have this function built in. My JR 8103 does not, but it's an easy mix.

If you don't have a computer radio, try some of the control horns that are threaded bolts with a plastic fitting. This way you can make fine adjustments on each side to get the moving together.