I’m using twin BB60Cs in something similar. And I’ve been through the same issues roughly two years ago.
(1) If you pass reference clock from one receiver to the other you need to properly set the reference clock frequency in both. When you will get proper time alignment you will see the drift. Discuss with Andrew about the special, kind of “dangerous”, API calls for properly setting the timebase information.
(2) Use IQ streaming instead of sweeping.
(3) Use an external trigger! A simple controller that can issue the sync pulses whenever you need and then align the samples in software. But be aware that there is a 4 samples pulse position imprecision at widest streaming bandwidth, as far as I recall. In my case the upper level algorithm automatically compensates it.
