Two 2.4GHz radio modules (XBee+Bluetooth) on same PCB. How to minimize interference?
I don't think it will be an optimum solution to place one module on top of the other. I'm not saying that it will not work because it probably will only with less reliable connection, lower datarate etc.

Bluetooth in particular has been designed to avoid interference (it changes channel 1600 times per second !).

In general it is always best to separate transceivers as much as you can for optimum performance. If you look at how antenna's on cellphones are placed inside the phone you should notice that they are usually separated as much as possible even for antennas that do not operate in the same frequency band.