This level of synchronization should be able to maintain a consistent synchronization over a period of time (i.e, the two sample clocks should not become further and further misaligned as time goes on). There will likely still be a small amount cycle-to-cycle jitter present between the two measurements, causing a corresponding non-zero relative jitter between the devices' sample clocks.
You cannot share clocks between 407x devices in a chassis; the best synchronization you can achieve between multiple PXI-407x devices is sharing a start trigger. Generally, this is more than sufficient, as the trigger will tell the DMM when to acquire their finite set of samples.
For more information on synchronizing please refer to Synchronization Explained