I could never get multiple instances to work, but the author suggests using the --remove-disabled flag in addition to what you are trying to do.
On a related note, I threw a NVDA 560ti into the first slot, and an AMD 7970 into the second slot, and I was not able to get --gpu-platform 1 working properly
Always defaulted to the NVDA card and did not recognize the AMD card. (cgminer -n identified NVDA as platform 0 and AMD as platform 1)
Windoze 7, CGMiner 3.1.0 FWIW