Idea about sync with Crossfader

i’m playing a song in deck A at 120 bpm - Loaded a song in deck B (original tempo at 100bpm)
If i use the crossfader for mix them , the tempo will go from 120 to 100 … Ok …but what about to change manually deck B bpm to 110 ??

It would be perfect if we can do the mix from 120bpm to 110 (not the original deck B tempo)

What if you set the bpm before you turn on sync?

sync is off until i touch the crossfader , The Crossfader mix only original tempos