We would see the heavy intermixture in one of the groups in that case. In addition, if one group was in homo sapiens captivity, then it is overwhelmingly likely that the other group would be found during those 50 thousand years (considering our propensity for migration).

Or slaves.

Masters have bred with slaves since the beginning of history. Abraham and Hagar, for example.

You don't really need slaves in a hunter and gatherer society (no agriculture, no construction, no mining = no backbreaking work that free people are loath to do), and you don't really have the institutions to keep them from running away.

As far as our observations of Stone Age people go, if they catch someone, they either kill them or make them a permanent member of the group.

Neapets