The Viper 15s have a problem with mixers. They have only a limited throttle range on normal stick movement, then they go beyond that and stop. This is not usually an issue with normal stick travel, but with a mixer you are forcing the controls beyond their normal range of travel at the extremes. This manifests itself as strange or no input at maximum deflection, such as full astern and full left or right. At this point either the speed control will stop completely or will start behaving oddly. The Mtronics Tio range have eliminated this problem and are fine with mixers, but the Vipers still have this issue hardwired into their chips.
The easiest fix is either a mixer where you can reduce the differential to say 50% instead of 100%, or set them up with the dual rate setting fitted to some transmitters, and dial that back. The only problem is that you need to set the speed controllers end points BEFORE doing any of this, otherwise, by setting the endpoints, you undo any gains as the Vipers now think that the reduced travel range is their normal full range..all very complicated.
Best bet, a couple of Tio 15s or anybody elses equivalents, plus that Action Mixer!