Will try this in few hours.
I have a Gtx 680 and gigabyte r9 280x. At first I had to play around with the --gpu-platform command to mine with the 280x, but at some point it got fixed, with a driver update or something. Anyway, now I don't need any special settings, cgminer automaticly uses the gigabyte, and cudaminer uses gtx 680 
I'm doing it too and it works perfectly with a GTX 770 and a R9 280x, but a dummy plug was required in my case. If you are using your nvidia card for games and stuff (which you should) you have to plug it to the motherboard's slot closer to the cpu. And then use --gpu-platform 1 in cgminer to select the ati one.
cudaminer/ccminer will detect only nvidia cards, but sgminer will detect all opencl devices (nvidia card included).
So for sgminer you need to run one time with option -ndevs so it displays all the opencl devices so you can select those corresponding to amd card then when running you will need to use something like
sgminer --gpu-platform #platform_number
So for sgminer you need to run one time with option -ndevs so it displays all the opencl devices so you can select those corresponding to amd card then when running you will need to use something like
sgminer --gpu-platform #platform_number
. I got 5-10% morre without doing anything. But the price of course 5% more 