I have a GTX1080 and I use it normally. Now I want to buy a RX460 for casual mining in the same PC (ethereum probably). Can I configure the miner software to mine only with the second AMD GPU? Thanks.
Honestly, if you mine some specific coins with GTX 1080 when you don't use it for gaming, I think it will be more profitable than using a RX460 at 24/7.
So, I recommend you buy GTX 1070 or RX 470/480 (570/580) and apply modded bios to get better earnings. Besides, with demand increasing for these gpus, mining with a single gpu will not give you much compared to people mining with at least 6 gpus.
Consider building a rig, but build your rig with safety concerns. Avoid children/pets at same room.