SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

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!

The Isolator webpage describes some of the interconnection problems with two timers. I would just use some homebrew circuitry with parts a Radio Shack would have in their stores, and be just as well off.