Guys, thanks for your further comments. Regarding dual-booting generally, I've had 7,10 and 11 all on one SATA HDD some time back. I've also had 7 and 10 dual booting from an M2 SSD. But I must have used GPT partitioning and had the BIOS set the same way, without knowing I needed to.
My current problem is that I want to add W10 to a W7 500GB M2 SSD and I've created a 120GB partition for that purpose. But W10 won't install there because the 500GB M2 SSD is MBR. Another factor is that I don't want to risk my W7 setup - I've spent too long getting it set up so it does everything I want, perfectly. I'm only installing W10 on my machine (desktop) to learn enough about it to help someone else with their occasional W10 issues.
The easiest way to solve my problem would be to install W10 on a separate disk, but it'll have to be on a spinner, due to the limitations of my Asus board. Also there's only one M2 slot, which is why I wanted to add W10 to the 500GB M2 SSD.
Incidentally, I read somewhere that Disk Genius can change a drive from MBR to GPT (or vice versa) without data loss, but it sounds a bit complicated. Maybe I'll just take the added spinner route.