What hardware are you using (other than the XP1)? It certainly does it with DDJ controllers other than the XP1.
Rekordbox DJ - Instant Doubles should also duplicate BPM of the currently playing track
When selecting instant doubles (my efforts were using the DDJ XP1 controller), the BPM of the already playing track is not duplicated unless SYNC is engaged for that deck.
I am coming from Traktor Scratch Pro, and the way that Instant Doubles works there is that all of the currently playing deck state is replicated in the other deck, including the BPM. I think that the BPM should be duplicated on the new deck as well. Remembering to hit sync is an extra unnecessary step which can eat away at transition time.
I currently use two turntables and a Traktor Kontrol Z2 mapped to rekordbox controls (which I know is unsupported).