Ripping two CDs at once is working for me. I didn't try it on 11.91, though, as I skipped that build.... but I'm guessing that this method should still hold true:
1) Change the default under Tools->Option to allow 2 concurrent.
2) Insert both CDs.
3) Click on Drives & Devices. (Actually highlight "Drive & Devices" instead of just dropping the menu down and selecting one drive at a time... When you do this, you should see the complete list of tracks from both CDs, but there won't be an option to rip above the tracks, as there is when you just select one drive at a time-- you'll just have an option to play.)
4) Choose "Rip CD" from the Action Window. The Rip CD window will open. Click "Rip"
That should rip both CDs. At least, it is for me.
Note: If you chose "rip and encode simultaneously" in your Encoder options, then MC will rip & encode track 1 from each CD simultaneously and will repeat this process all the way through the CD, so both CDs will complete around the same time. However, if you did not choose this option (for instance, if you choose to Normalize before encoding instead), MC will only rip one track at a time, from the first CD to the last, but it will rip quickly and will encode the past ripped files while it begins ripping each subsquent file.
Again, this is how it's workign for me. Others may be expereincing something different. The process seems to be working quite well, though.
The only error I've been experiencing with this with MC 11.1 is that:
When I'm using "Normalize before encoding" and I have MC set to eject the CDs when ripping is complete, the encoding process often won't complete or MC will freeze before completion.
So I changed MC so that it doesn't eject the CDs when ripping is complete, and instead I manually eject them when the encoding is complete.... and that solved the errors. Of course, maybe the error shouldn't be happening in the first place...
(Specs: When ripping I encode to MP3 VBR files, using the standard MC Mp3 encoder set on high quality and fast mode; I typically don't keep rip logs.)