Basically, queued fast sweeps just go. Unfortunately there is a little bit of uncertainty each time the LO changes. We check to see if it has settled, and add a few microseconds delay if it has not, so even if they started in sync it would not last.
We could add this to our list of feature requests. Most likely this would be a feature we would add to a future product (multi-channel receiver) due to the complexities of synchronizing two standalone devices.
If you could scan your spectrum more slowly, 40 MHz at a time, you could use I/Q streaming and a shared external trigger to get them sync’d within a handful of nanoseconds.
