Hello ! That is a good experimentation, and having a working multi-boot can be a great added value
From the Fairphone forum, it seems that the order of installation could be crucial! Quoting @z3untu :
It 100% doesn’t work as a second rom right now. It could maybe work, if you install EFIDroid (from Android), create a second rom (from Android), flash Ubuntu Touch (via recovery/fastboot), put the boot.img from Ubuntu Touch into the correct location (don’t know where right now), flash the boot.img from EFIDroid and then theoretically it could maybe work.
Ubuntu Touch as a secondary rom isn’t supported yet but I meant that it might work as the primary rom.