I use an old PC with BIOS, and about UEFI i don't have any experience.
If you use two HDD, one with Windows OS and the other with Linux, then if Windows HDD is selected to be booted first you will not see the Linux Option.
If you select the second HDD to be first on system boot with grub installed on it you will se both systems Windows and Linux but with the Linux as first option. If you want to change the boot order later, as I know that this can be done with Grub Customiser but i don't know how it works on Solus.
Hope i have explained it well.
The second options is what i have done, in my PC.