www.makemkv.com

Hiya,

Is it feasible, from a disk writing performance point of view, to rip from a built-in DVD drive on a laptop while simultaneously ripping from a USB connected drive (or multiple connected drives)?

Will MakeMKV even allow this?

You should be able to start multiple copies of MakeMKV with few catches. catch #1 - both programs will fight for disc for a short time (one would read it for conversion and another will try to scan it) so it may take some time to open a disc. Catch #2 - MakeMKV uses heavy memory buffering during conversion and by default allocates 1/4 of all available physical memory. so if you laptop has less than 1 GB of ram you might have to adjust read buffer size in preferences.
with points above in mind you shouldn't have any problems launching two copies at the same time.