For quite a while I've found that the much easier answer is to have a physical drive per OS and make sure it's the only drive connected during install, or at least one for anything that doesn't play entirely nicely with multi-boot. Obviously there's downsides to that, buying another drive or you might be using something like a laptop which is less friendly to extra drives, dis/reconnecting M.2 drives isn't as trivial as SATA either.