I had this same problem with my four USRP2 boards, each with a WBX
daughterboard. The solution was to use the 10 MHz external clock
reference input. I have an HP signal generator which has a precise 10
MHz sine wave output on the back. I split this signal four ways and
input it into each USRP2. Now the clocks on my four USRP2 boards seem to
be synchronized, so if (for example) one board transmits a tone at
903.55 MHz, another board will see it at 903.55 MHz.
Please let me know if this helps.
Steve McMahon