Oh my ... Can we not confuse the OP? As I read and visualize the OP's situation, he has ONE timer but sprinklers in the front and back. That suggests only ONE power source ... which is the most common configuration for residential applications. If he has TWO separate timers, they would each act as a separate power source BUT the common wire would NEVER be connected bewteen the two timers.
The valve manifold can serve a wide variety of zones, both front and back. The purpose of the timer is to systematically power the valves. The limiting factor is the number of zones which the timer can accommodate, right? If it has power points for six zones it does not care "where" the zones are (front or back). Each of the valves has two wires with one always being connected to the WHITE common wire. The other wires are connected on a color coded basis.
CONCLUSION: If he has only ONE timer, this is an easy problem to solve. If he has TWO timers, there is no reason to connect the common between the two timers.