Reply to Syncing Separate Mixers

Me and a friend have recently been starting to duo Djing. He has the DDJ-SX and I have the S2 (Soon to be S4) we are both connecting into the same speakers, and are having some issues syncing up to each others songs by ear. Is there anyway we can connect so we can both sync to each others songs BPM when transitioning to a new song by software or other hardware?

Thanks!
Mike