There isn't a way to limit total throw n the 7C transmitter. Only the 12FG, 12Z, and 14MZ have settings for the total throw allowed for each channel. Those transmitters have settings for the normal end-to-end throw of the stick, switch, or slider, and then a total throw allowed when mixers are involved.

You'll have to set up the controls so that when you have the mixer to the extremes, the two channels mixed don't over drive.