Problem with USRPs (phase offset)?

[USRP]

Hi                        I used this configuration (2 receivers (USRPs) and 1 transmitter (generator of sin wave)):

I did time and phase synchronisation .

the problem:

The received signals by the 2 USRPs are not superposed !??  
how can I do to supperpose the signals received by the two USRPs??

please help me

Best regards