Unfortunately, with the standard release of Audacity the answer is no. Audacity can only access one device for recording, and one for playback.
If your MOTU devices work with ASIO drivers, then you may be able to use both devices simultaneously with Audacity if you build Audacity from the source code with ASIO support. (ASIO support can not be built into Audacity as standard because Steinberg’s license does not allow it in open source programs).
Running multiple instances of Audacity is not advised. In some circumstances it may work, but in most situations it will lead to bad data, system crashes, and other undesirables.
With Audacity you can record up to 16 tracks simultaneously, providing that the hardware gives Audacity access through a single device driver. I’m guessing that your MOTU can each record 8 channels - If each device uses a single driver for all 8 channels, then Audacity will probably be able to record those 8 channels.
Audacity can only play back 2 channels (stereo) so you will not be able to take full advantage of the multi-channel output.
Of course, because Audacity is multi-tracking, you would be able to record the first load of tracks, then add more tracks in subsequent “takes”.
On Windows, “Reaper” is a good, cheap (not free, but free demo & low very purchase price) multi-track recorder that uses ASIO. Reaper and Audacity work well together, using Reaper as the DAW, and Audacity as the wave editor.