Follow up on multiple simultaneous MDRP instances on the same computer: I tested, and it works, but not smoothly.
The "unsupported" part is that if you have two MDRP instances open when you insert the first DVD, they're going to fight over control of it and both try to rip it simultaneously, which obviously isn't useful. However, once an instance is actually underway ripping, it will ignore any further DVDs inserted, and any instances launched after the disc rip is underway will ignore that disc. Not sure if there will be any side-effects in terms of how it keeps track of what's been ripped (maybe MDRP just checks the destination folder, rather than a database?), but the rips were both ok when I skimmed through them in DVD player.
So, you can do this: Start one instance, stick in the first DVD, and start the rip. Now, start the second instance and insert the second disc. Both will be ripping a different disc now. If one of them finishes before the other is done, it will sit and wait for a new DVD, and if you stick in a DVD while the other MDRP instance is still working, the free one will grab it and you're good to go. So basically, once you've got it going, as long as both rips never finish at the same time, it will work. Since DVDs are of irregular length, it probably won't be that hard to keep both instances running smoothly so long as you're reasonably prompt about swapping discs when one finishes. Even if both coincidentally end up finished at the same time, you just need to quit one manually to get back on track.
Bottom line being that it's pretty doable if you really want to speed the ripping process.
I'm not going to bother, since a single disc at a time is working fast enough for my taste (and there's no mental effort involved in making sure I don't put the wrong disc back in the wrong case or get things out of alphabetical order). If I were to do it I'd probably use my mini for the second rip, just to eliminate the possibility of any glitches.