** Our FP+ for BTMRR worked on Space and 7DMT ** when we were there in October, not sure why or how but it did.
And vice versa.
We had FP+ at 7DMT and it was down during our stay for like 4 hours, during our FP+ time and our FP let us onto BTMRR during the 7DMT time slot....
Disney does this when rides go down. They open FP usage to several similar level rides. Ordinarily, FP is for one ride only, not interchangeable such as when rides go down.
(Yes- one of us would have given her our magicband and the other parent would have just had to wait outside). I stalked the website constantly. Eventually an overlapping one popped up and I grabbed it. It doesn't matter if you just have 5 minutes of overlap as long as you have some. Actually, they will let you in 5 minutes before your FP+, so even if there was a 5 minute gap in between them, you would be okay.