There is always a chance you break something, so only continue if it's no big deal installing the whole disk from scratch after messing it up!
Chanting "backup, backup, backup"
If you're current setup is this:
- sda
-- sda1: 512mb (Solus boot)
-- sda2: 240Gb (Solus root)
I would use a partition manager (gparted, disks, etc.) to shrink the sda2 partition to create two new partitions like so:
If you use the gui of such a tool, it should be pretty self-explanatory.
- sda
-- sda1: 512mb (Solus boot, Fat32/exFAT, boot/esp flags set)
-- sda2: 200Gb (Solus root, ext4)
-- sda3: 512mb (Ubuntu boot, Fat32/exFAT, boot/esp flags set)
-- sda4: 40Gb (Ubuntu root, ext4)
The order can be different and I rounded the sizes a little, so don't assume it should be exactly like this. I am not sure what distro you want to double boot, but some may not actually enable you to install the bootloader in a custom partition and simply use the first one they encounter (or all).