Just in case you want to install and try UT, you will need this older version of multirom TWRP (this is NOT a zip but an img file):
TWRP_multirom_hammerhead_20150630.img

And if you would like to put back the newer one (this is NOT a zip but an img file):
TWRP_multirom_hammerhead_20160210.img

You can flash both as an img from recovery. Switching between these to recovery images is safe any time and will not impact your main or secondary roms. The MultiRom menu option is located at a different place in both and only with the older (or newer?) multirom recovery you can install UT from the apk.