Start cgminer only on the second card HOW?
to maps gpus i use this script:

cgminer -n option

Timeout /t 3600


Make it a .bat file and put it in the same folder as you cgminer, then you can tell witch gpu that is 0,1,2... 

but you have to turn off power saving in windows to prevent you 280x form sleep, if you don't have a dummy-plug or an screen put to that device.

I used this guide for all start up information's: http://www.cryptobadger.com/build-your-own-litecoin-mining-rig/

Regards

EDIT: and when you use the -d command remember to have a space before the number.