Jumpy Only one of the disks have a (shared) efi boot partition. But I'm not sure if you could have one on each of the disks.
Of course you can.
The method I described sets up two independent EFI partitions, the Solus EFI partition on the Solus drive and the Windows EFI partition on the Windows drive, intersecting only at the UEFI/BIOS Boot Menu, as described.
You can also (and it is the more common method, I suspect) set up an EFI partition on each drive by setting and resetting EFI flags during the installation process so that the EFI partition on the non-target drive is not recognized during installation on the target drive. Others have described the flag-set method of dual-boot, dual-drive installation on other threads. I don't use the flag-set method, but it seems to work fine for those who use it.
The reason that I elect to remove the non-target drive during installation rather than flag-set is that the flag-set method is more complicated and more prone to installation error than the method I described. But each gets to the same point -- two independent OS installations, intersecting only at the UEFI/BIOS Boot Menu level.
I've been dual-booting for years and years, and have not had good experience with shared-EFI partitioning. Sooner or later, I need to replace/reinstall one or the other of the two OS's, and at that point (at the reinstallation point or down the road) the shared-EFI partition has encountered problems. No every time, but often enough to become an issue.
Setting up dual drives, each with an independent EFI partition is extremely stable, and allows you to install/uninstall/replace/change operating systems on each/either drive at will, without affecting the EFI partition for the other OS at all. A shared EFI partition does not allow you to do that without running the risk of partition corruption.
That is what led me to dual-boot, dual-drive, independent-EFI installation about four years ago.
However, dual-drive, dual-boot independent-EFI installations are most suitable for users who use one of the two operating systems as primary, using the other operating system only secondarily. In my case, for example, I use Solus about 90% of the time, and Windows about 10% of the time, using Windows only for specific purposes. Because of that use pattern, it is most convenient for me to have the computer boot into Solus as default, requiring UEFI/BIOS Boot Menu intervention if and only if I want to boot into Windows. I don't have to select at each boot, and don't want to. If I were using Solus/Windows 50/50, or flipping back and forth between the operating systems 10-15 times a day, having a startup boot menu show up at each boot (the method you described) would be more appropriate.
Linux provides many ways to skin the cat, and that is a strength in my opinion. I'm glad that the shared-EFI method works well for you. I had a different experience and use the independent-EFI method. For me, the independent-EFI method's strength is its conceptual simplicity and remarkable stability. Others elect not to dual-drive dual-boot at all, but single-drive dual-boot instead. I've never used that method, so I can't comment on it. Whatever works best for a specific use case is what works best for that use case.
