If you are running UA-5 > optical > H120 in 2 places, you are using the UA-5's clocks. I've had good experience doing that, generally they are so close I don't need to shrink or stretch, just align. Temperature could be a factor. If one is in a hot bag, and the other on a cool bench, that could make the clocks differ.
To page's point, I find my MR-1 is the one with the drifty clock, and it's not linear.
There are a bunch of different ways to stretch/shrink one. My suggestion is this:
- align the front, see how much they are off at the back (maybe like .300 seconds), and you want to shrink the bottom one.
- Then in Audacity, highlight that track and click "Effect -> Change Tempo".
- At the bottom of the screen it says "Length (seconds) from xxx.xx to xxx.xx". Let's say it's 5000.00 seconds. If you want it shorter by 0.3, change the right box to 4999.70
- Hit OK and let it churn. Go line up the front again if necessary, and see how the back lines up.
- If you don't like it, edit -> Undo (twice) and repeat with a little different number.