It really depends on what you are trying to do. You can absolutely stick them both in the same system and they will show up. But depending on the application you may need to change the order the cards are in (which slots), change the order of the drivers installed, may need to use different video outs.
I tried using a R7 370 with a GTX 960 for folding, it kind of worked but not really. The system would boot, see both cards and would start folding but would always fail on the R7. I think this is an issue with the card itself because it didn't even work when I put it in a system by itself. However when I tried putting an RX 480 in the system started having issues with not booting at all. I changed the cards position, driver installs, used older drivers newer drivers and everything in between but no go.
It could work, but you may need to put a ridiculous amount of time into making it work. Cards from the same manufacturer in the system works much better even if they are different gens.