Apparently, AMD's newest Catalyst 15.7 WHQL driver not only adds support for AMD Freesync on Crossfire systems but also allows users to Crossfire AMD Radeon R9 390X with Radeon R9 290X graphics card.
While these two are pretty much identical graphics cards, since Radeon R9 390X is a Radeon R9 290X rebrand with higher clocks, it does not come as a big surprise that Crossfire works between those two graphics cards, and should be possible between Radeon R9 390 and R9 290 as well.
We are not sure if Crossfire between these two graphics cards was possible on earlier AMD Catalyst driver versions, but it works now as the user on Videocardz.com even managed to get a clear score in 3DMark.
Source:
Videocardz.com.