For PureOS I’d create only three mandatory partitions: ef02 one (+32768), boot (+1048576) and home up to the half of the one of yours HDDs. Yet to tell Calamares that it should use three partitions as a whole (8300 for 2nd and 3rd one) we need to create fourth to reserve the space for the latter partitions. For first two I’d stay with gdisk /dev/sd[x] command, but for other two (before you click on Install PureOS icon) you might choose to use Gparted (search for Desktop icon) afterwards as somehow easier to find midpoint of your HDD (half of it for the first three partitions). You don’t need to necessary format fourth partition (don’t use it during PureOS install … just writing something that you know already), but creating it under GPT will expedite your next installation).
But before you start to install (might be related one of your posts here) check please the third partition with:
sudo parted /dev/sd[x]
(parted) align-check opt 3
3 aligned
(parted) q
As I find Debian-Installer up to the my taste I’d recommend to install Debian with non-free firmware as it will recognize all needed for your WS (so you can double-check PureOS non-free add-ons). Just after apt upgrade please execute apt full-upgrade as well. End this first boot (after installation) with apt autoremove and apt reboot. Third and fourth options for this BIOS MBR mode HDD perhaps might be Devuan (just because you asked) or How-to: Gentoo Linux from @lperkins2 (both are non-systemd).
For the second HDD with WIN10 (under UEFI mode) up front as @Gavaudan recommended (by planing to have /dev/sd[x]6, plus others, from the middle of HDD), the best option is to go with some other distributions, but if you want to make things as simple as possible (keep/make your PureOS user experience advanced than use the same Debian image as above linked or take Pop_OS (if @reC don’t mind). Also, Ubuntu was always very reliable distro (learned a lot from it, but after 14.04 LTS, or even earlier, I lost my contact with it completely).