Using the XBee in conjunction with other shields on the Arduino shouldn't present a problem, as you only need two pins to communicate between the Arduino and the XBee. They can be any two pins.
Having the two devices push data to the collector might be a problem. Having the collector request data from the two devices would eliminate that problem. (Hey, Joe, what do you have for me? (Tom ignores this request) Hey, Tom, what do you have for me? (Joe ignores this request)).
Collecting the data from the remote stations, and adding its own data, won't be a problem for the base station.
Sending the data via GPRS/GSM will be the hardest part.