It looks like you are trying to keep this to only a few modules, but with a CM15A ( and ActiveHome Pro with smart macros of course), the motion sensors, and an appliance module, this could be done very easily. Using smart macros, have the macro handle the timer and the meat of the operation. You could have the macro set up so that if the first motion sensor sensed motion, it would trigger the timer in the macro. When the second motion sensor sensed motion, it would reset the timer. If you set it up so the motion sensors triggered the macro, and the macro activated the appliance module, there would be no worry of either motion sensor prematurely shutting off the appliance module.
Unfortunately going with this solution would mean spending a bit more on the modules and software.