USRP2 reference clocking

Hi,

     I am trying to synchronize two USRP2s. I connected GPS 10MHz clock
to ref clock (to SMA) of 1st USRP2, and used the MIMO connector from 1st
USRP2 to 2nd USRP2 as given in
the http://gnuradio.org/redmine/wiki/1/USRP2GenFAQ#What-are-the-electrical-specifications-for-the-10MHz-reference-signal.
I noticed the clock drift is still there and no synchronization. Couple
of questions.

(i) Do I have to do any modification in the source code and in the
firmware?
(ii) Is there any simple program to check if USRP2 is locked to external
ref clock.
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Any help is appreciated.

Thanks
Krishna S
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