iPad Air 2; iOS 10.3.3 -> iOS 15

[nocontent06]

My iPad Air 2 is running iOS 10.3.3. I don't wanna update it, because i can't downgrade it back then, if i wanna have iOS 10 back. So i wanna have iOS 15 and iOS 10 dual booted, or iOS 10 and iOS 15 dual booted. But the problem is, iOS 15 tells me "Can't restore, Filesystem isn't mounted".

What's your advice?