A little addition to @sailor's very useful post:

If you would like to install Ubuntu Touch with multirom on a Nexus 5 you will need:
- a modified TWRP v33: https://forum.xda-developers.com/goo...m-v24-t2571011
- the modified MultiRom apk: https://forum.xda-developers.com/sho...&postcount=580
(little more details about this can be found here: https://forum.xda-developers.com/ubu...blems-t3409123)
- a kexec kernel: you can find downloads on the same link where the TWRP v33 (https://forum.xda-developers.com/goo...m-v24-t2571011) or use what @sailor wrote

Once you have these all, you need to install the MultiRom app on your rooted Android (main rom) then open the apk and select the version/flavour of the UT you would like to install.

Note: With this TWRP (v33) the flashing zip file on any rom does not work. If I need to flash a zip file on a rom, first I flash an older TWRP twrp_multirom_hammerhead_20150630.img (can be downloaded from: http://bin.ge/dl/247675/TWRP-multiro...50630.img.html) then I boot into it to flash the zip file.
Note 2: the older TWRP is not compatible with UT, so if you would like to install a different flavour or update the current one, you need to go back to TWRP v33 again.
I hope this all does make sense.