980Ti and 780Ti run together
Can I run 980Ti and 780Ti together with latest hashcat? 

I got this massage:
* Device #2: This hardware has outdated CUDA compute capability (3.5).
For modern OpenCL performance, upgrade to hardware that supports
CUDA compute capability version 5.0 (Maxwell) or higher.
Device 2 is outdated, you need to upgrade if you want things to work without that error.... pretty self explanitory
So there is no way to run both cards together?
as far as I know this is just a warning, not an error.

Of course it would be better to use more modern/newer GPUs, but as far as I know it is still possible to use these 2 different models together.

Why do you think that it is not possible to run them together? Did you actually test it ? If yes, what was the output ?
(and again, a warning is not an error... in this specific case it is just a hint that a newer GPU would be much better)
Thanks philsmd. It was just a warning.
Both card are running fine.