When you say "nearly the same time" how close do you need the arrival time to be? I ask as I was trying to set up a similar setup using XBee's and the synching was not good enough, it was off by +/- 1-5 ms and I needed about .1 ms resolution. The timing difference adds up from things like the serial interface lag on both sides among other things.
I am now redesigning the system with GPS modules at each units that output a pulse per second that each GPS will receive at very close to the same time, I think the differences are on the order of nanoseconds. The PPS will trigger an interrupt and the MCU will reset the system timer and record a time stamp, then when an event happens the remote units will send back the number of timer counts and the last time stamp.
Not all GPS modules output a PPS so if yo are going this route make sure the GPS modules you buy do output a PPS.
wade