I was able to measure this.
Yellow: This is from a SRS FS725 that warmed up for ~10 minutes before starting the test. The locked light was on, but it might improve stability if it is on for a few hours/days. It is also coming from a distribution amplifier that doesn't have the cleanest waveform
Green: this is from a separate SRS FS725 that has been powered up for probably a few weeks. The output is taken directly from one of the instruments 10 MHz output.
There is no perceptible drift or jitter, the waveforms look completely stable on the scope. I took two photos 8 minutes apart to show the slow drift: In that time, the green trace has slewed approximately 30 ns, or about 62 ppt. That is well within spec, the accuracy at shipment is supposed to be +/- 50 ppt, and aging increases that further.
Since there is some definite offset, I don't think there can be injection locking, this is the real stability. I will try again after the first source warms up for a few hours.