RoWin7, I recently completed
a video that you might find helpful. You don't have to follow the same technique, but some of the background info may be useful to you in understanding the issues and deciding how you want to do it.
The first thing you'll want to do is settle on Legacy boot or UEFI. You could have one Legacy boot disk and one UEFI disk, but it would be a hassle to have to go into your BIOS and reset the boot mode and Secure Boot every time before switching to the other OS. So I recommend choosing one or the other and having both OSes boot the same way.
If you settle on Legacy boot, you can put Win7 [System+Boot] on one disk and Win10 [System+Boot] on the other, and choose the boot disk from the BIOS boot menu. Both OSes would be completely independent, and you could remove either disk and the other would still boot. Note from my video that if you use Macrium Reflect it's trivial to convert Win10 from UEFI boot to Legacy boot.
If you settle on UEFI boot, you'd need to install Win7 as UEFI boot on one disk and keep Win10 as is, on the other disk. Again, both disks would be completely independent. Instead of choosing the boot disk from the BIOS boot menu, you'd choose the OS. That means you'll need to make sure the second OS is added to the BIOS boot menu. I don't have much experience doing that so don't know what pitfalls there may be, but I don't think it's hard -- certainly no harder than getting Win7 installed in UEFI mode in the first place.
Personally, I don't understand the reluctance to put two OS partitions on the same disk, but if you're against that it's certainly possible to do it the way you want. All the various possibilities are just a matter of how much effort you're willing to put into setting it up.
For comparison, if you were to follow the technique in my video, you would:
- make a backup image of the Win10 system (at least the OS partition, but no harm imaging the whole disk);
- switch BIOS to Legacy boot and install Win7, leaving unallocated space for Win10;
- restore Win10 OS partition to unallocated space;
- use Macrium's "Fix Windows Boot Problems" tool to rebuild the BCD.
(This assumes the boot disk is no more than 2 TB and the computer supports Legacy boot.)
Thereafter, you'd choose the OS from the BCD boot menu, not the BIOS. You don't have to hassle with getting Win7 installed in UEFI, and you don't have to hassle with setting up a multi-boot manager because Macrium does that for you automatically. However, the two OSes would be on the same disk.