I want to do the exact same thing and haven’t figured out if the following is the right way:
Modify the current btrfs volume to use only the SSD disk (in your screenshot both disks are selected). Use that volume for / .
Then create a new btrfs volume (click the volume name, a dropdown menu appears with the option, I believe) and make it use only the hdd disk. Use that volume for /home.
In theory this should work, but I did not go through with the installation, because I dualboot and was afraid of messing something something up. If you do try this, let me know if it worked.