one needs to be the master. Do you have one on? (it will flip to the other player "automagically" whenthe master track is done)
Be aware that the beatgrids can cause embarassing moments if you use sync though. I have a few tracks that at the end will have irregular
beatgrids or slower... if you don't stop the master early enogh or disable sync, you will end up in a mixed track that all over sudden drops
several BPMs.
My 8yo girl here uses sync to quickly get the tracks at the same speed and then she disables sync due to above mentioned problems.