Although multiple controllers on one system aren't that uncommon in my area, I do not recommend them either. To many complications, variations and etc.. But if you still desire to go this route, this device here will simplify things.
www.transitionalsystems.com/isolator.htm
or
www.transitionalsystems.com/piggyback.htm
Of coarse the real chalenge is finding a distributor that caries these! Good luck!
Josh
Irrigation /Landscape Lighting / Pump and Well Specialist