My question is what happens if I run two instances of bminer one for ZEN and other is for BTCP ? both on same pool (suprnova). But without configuring GPU usage. Does one intercept other ? can I fully utilize my GPU power ?
They're different pools, but both owned/run by Suprnova.
Yes, you can mine both ZEN/BTCP pools, you just need to specify GPU devices for each running instance of the mining software using whatever the device flag is. Otherwise, the miner will try to use all GPUs for both running instances.