I very much want to try out Sailfish OS, but would like to be able to try Debian at the same time.
Check some other recent threads on the Sailfish chat, or for that matter the partition tool itself, you'll find that Planet have decided against the 'stowaways' form of multi OS hosting thus meaning that you can only install Android+One other OS with the flash tool. The options disappear as you select them from the drop-downs on the partition tool.
This is what I was referring too with my multi-OS comment above, if your game for it then you could try installing the Android+Sailfish as from planet (get happy with it and decide how much space you want to keep for it), then resize (shrink) your Sailfish LVM volumes, and get back to us and we could try making a LVM compatible version of Debian that you could then 'dd' to a boot partition/root unzip from within sailfish to a new debian volume in the space made available by shrinking the sfos volume. First thing I'd like to know is what the sailfish volume names/size etc are. The idea is that our debian LVM release (probably TP3) would have different names from sailfish and so you could then install either A+S or A+D from planet then later do the resize and copy the other one (or Kali/UBPorts/Other future OS) across later for multi-os usage. Thus allowing OS changes without resorting to the flash tool.
I'd also be interested to know if people would want to have things split so that maybe a stretch root and a buster root are available with a debianhome that they both mount so that you can share user files between os's? (could possibly also extend to get sfos/kali to mount the same root?) Or is it best to keep things clean and just allow users to mount the other homes as desired, there are some bits of software that change their file formats between versions and so for example Telegram Desktop gets confused when you launch the stretch version after launching the buster one (you just have to login/2fa again).