I have had trouble dual-booting on separate drives in the past because, sooner or later, the boot managers get tangled.
I've found a simple (if tedious) way to dual-drive, dual-boot successfully. Here is what I do:
(1) I back up all the data and anything else I want to keep (e.g. Windows installation executable files) I want to keep onto an external USB or external drive.
(2) I create a USB installer for Windows using the Media Creation Tool (Windows 10 or Windows 11, as the case may be).
(3) I create a USB installer for Solus (in my case using Rufus on Windows).
(4) I boot into BIOS (F2 on Dell computers) and remove all boot managers (Windows Boot Manager, Windows Boot Manager, Grub, whatever). I also make sure that the computer is set up correctly for a Solus install (UEFI, Secure Boot disabled, AHCI rather than RAID, and so on).
(5) I remove** all but one drive from the computer.
(6) Using the Media Creation Tool USB, I do a "destructive" clean install of Windows on the drive in the computer. By "destructive", I mean that I use the "Custom" install option for installation (part of the Windows Media Creation Tool USB installation process), deleting all partitions on the drive as part of the installation process. As a result, Windows installs on an absolutely clean, blank drive with no prior partitions, including boot partitions.
(7) I get Windows set up with drivers and whatnot so that the computer boots properly into Windows with no hardware issues.
(8) I boot into BIOS and check the boot manager to make sure that the only boot manager is the Windows Boot Manager.
(9) I remove the Windows drive from the computer. I put the other (soon-to-be Solus) drive into the computer. At this point, I again have only one drive in the computer.
(10) I install Solus on the drive in the computer, again using the clean "destructive" installation technique (in Solus, the option is to "Completely erase the drive ..." rather than "Install alongside ...") so that I am installing Solus on a clean hard drive.
(11) I get Solus set up with drivers and whatnot so that the computer boots into Solus with no hardware issues.
(12) I boot into BIOS and check the boot managers (should be Windows Boot Manager and Linux Boot Manager) are in place, and I set whatever boot manager I want as default (in my case, Linux Boot Manager for Solus) to be the default bootloader.
(13) I put the Windows drive back into the computer. At this point, I have two drives (one Windows, one Solus), each with an independent, completely separate boot partition, not seeing each other at all. The BIOS will boot into the default OS (in my case Solus) without intervention (F12 on Dell computers) but will boot into the non-default OS (in my case Windows) using F12 or whatever other key accesses the BIOS boot menu. Because the bootloaders (Windows and Solus) are on separate drives and do not interact, the bootloaders don't corrupt each other or get tangled. In essence what I am doing is building two computers (one Windows and on Solus) intersecting only at the BIOS boot menu.
(14) I boot into Solus and set the OS up the way I want to set it up (apps, data, appearance, settings and so on).
(15) I boot into Windows and set the OS up the way I want to set it up (apps, data, appearance, settings and so on).
(16) I start using the computer.
(A) I have never tried to set up a computer this way with any distro other than Solus. So my comments above are limited to a dual-boot, dual-drive Windows/Solus installation.
(B) I install Windows first and Solus second. I have no reason for this, other than habit. I can't see any reason why it would not work the other way around (Solus first, then Windows), but I have no experience with that method.
(C) If you set up Solus (rather than Windows) as your default boot OS (as I do), then you will need to be careful going forward handling Windows updates and attendant restarts, being careful to always use F12 to restart into Windows after an update. If you don't, the BIOS will default to Solus, opening it, and the Windows restart process does not proceed normally. My own practice is to use the "Update and Shut Down" option instead of "Restart" in that situation (see NOTE (D) below) because it reduces risk of error.
(D) I disable "Fast Start" (or whatever it is called these days) in Windows, so that power off doesn't try to save the system state. I don't have a technical reason why this is necessary in a dual-drive dual-boot environment with separate bootloaders (as described above) but I prefer to have Windows completely shut down when I shut it down because that is what I want it do do when I "Update and Shut Down".
** In the case of an M2 SATA or NVMe drive or a SSD or HHD that is installed into a slot (as in the case of a laptop or micro desktop), the drive will need to be physically removed. In the case of a SATA SSD or HHD in a larger desktop, the drive can be "removed" by unplugging the cable to the motherboard.