You're got quite a few variables in play here, so I would suggest starting with just the new card in a 16x slot and check out system performance for:
- timeline playback - are you losing any frames using your actual timelines (media, filters, etc.)? You can check with full resolution, 1/2 resolution, etc. too.
- render output - is it OK for your needs? In general multiple GPU cards can help Premiere render speed quite a bit if you are outputting to DVD format, but I don't think a second card will help at all for other formats (Blu-ray, etc.)
And if you are happy with the way things work with just the new card, run with it!
- you only have 24 lanes to play with so putting in both cards will likely downgrade both GPUs to 8x for a dual GPU setup
- using the lesser card to connect to your monitor (and the faster card for CUDA) could potentially help playback if you are getting lost frames with the single card; you would definitely have to test this on your exact system - if the single fast card does not work well for the tests mentioned above
- do you have enough watts in your P/S to support both cards comfortably? There's nothing more frustrating than GPUs not getting enough juice - dreaded blue-screens, etc.
- is noise a factor? In my builds, the GTX 900 series stuff is really silent, so adding prev. generation cards could potentially up the noise level of your whole system
Thanks. All good points. I have XFX 850W Core Edition Single Rail 12V 70A PSU. It should be able to handle the power requirements. I guess I will have to test it with a single and dual cards and see if it helps or hinders performance. The funny thing I assumed people have already tried it, but I could not find any reports on using two different GPUs vs. single here or anywhere else.