Why are FDRs and CVRs still two separate physical devices?
You probably wouldn't believe the effort needed to certify anything that flies on an aircraft. This is a multi-year process, especially if it is so much security-related as the CVR and FDR. And it needs to be done with all national authorities of your intended customers.

This is a case of good-enough. The advantage from combining both is real, but so small, that nobody so far thought it worthwhile to go through the certification process. And regarding redundancy, transmitting the recorded data to some ground station would be even better than recording twice on the same aircraft. If that would have been installed in MH370, most of the search effort could have been saved.

I expect something to happen with the transmitting of data, and then both devices could conceivably merged into one.