Start in fastboot mode and run fastboot getvar all from your host and look for current-slot. It will be either a or b
EDIT: fastboot getvar current-slot may be easier.
Then use fastboot --set-active=a or fastboot --set-active=b to set the active slot to the opposite slot as you found it currently was set. Then follow the instructions for installing UT and it will install to the slot you just set.
EDIT: The latest UBports Blog mentions that Lineage encrypts the home directory by default which means you will not be able to share data between OS's and it could cause other unknown issues.