We're experienced 2up crashers as well. Typically when we're rolling along on rough ground Sabrina dosen't hold onto me but the grab bars on the bike. This leaves me free to stand up on the pegs and do the driving that needs to be done. This also tends to keep the weight further back so we're not loading up the front as much in loose terrain.
When things go sideways Sabrina has always just "stepped off" but she's never been involved in a highside. When she gets off the bike she's often standing or stumbling. I tend to stay on and "ride it in" in an attempt to save it. Oddly enough she's never stepped off when it wasn't going to fall down anyway. I asked her how she knows to step off and to what side and she replied "I just feel it in my bones."
Our over all suggestion is to have Jill sit a bit further back when riding on the dirt with her weight on the pegs as much as possible, if she feels the bike pass "the point of no return" just stand up and step off.
We also stop quite often when things look hairy and I'll ride through it and Sabrina will walk.
For Mike, if you want when you guys get to Guanajuato we can practice some simple riding positions which help a lot in putting your weight in the right place and keeping the bike upright in less-than-predictable terrain.
Looking forward to meeting you