Im pretty sure you can cause i was gonna crossfire a 290 with a 390 the problem with it is gonna be its gonna knock your Vram to the lowest card i think the 280x is 3gb and the 380x is 4gb so itll bump you down to 3gb
edit: nvm think im wrong its different architecture
280/7900 = Tahiti
285/380 = Tonga
390/290 = Hawaii
Dont both 280x and 380x use the same drivers?