Why can't you?
Take an Afghani male cross to Chem 91, then find a male from those seeds and pollinate the Chem 91.
That’s not a legitimate bx though. For it to be a proper back cross you’d have to have a male from the same strain.
So in your example the chem 91 is A and afghani is B
Cross A & B results in AB f1
AB f1 (f) x AB f1 (m) = AB f2
AB f1 (f) x AB f2 (m) = AB bx
Now If you take a second f1 female and find a male from that seed and pollinate the first female it wouldn’t Technically be a bx.