Dual mining is maximizing the work load of the GPU by mining two coins at the same time, but it's not as efficient as mining the coin by itself. You are getting more hash because the Nicehash miner isn't a dual miner. With the Claymore miner, you can adust the workload intensity of the secondary coin up or down using the -dcri parameter in the command line options or using th '+' or '-' keys. The higher the -dcri intensity, the more hash the secondary coin will have. The goal is to find a number that maximizes the hash of the second coin, without affecting the ETH hash, but the secondary coin hash will never be as fast as mining the coin by itself.