There is no built-in option to select particular gpu's only yet. The default scripts select all available Gpu's.

For example you could use ETH_EXTENSION_ARGUMENTS="" and supply extra arguments for Claymore miner (consult claymore how to select gpu's), but if you want to mine ethash with gpu's 0-3 and equihash with gpu's 4-7 You would need to supply appropriate arguments to both miners involved and you will need to use individual PL, Core and Mem settings because different algos require different settings.

Hopefully we will implement some simpler settings for selecting particular cards, dual/triple miners and algos in future versions of nvoc