Why only dual mining only AMD cards and not Nvidia cards  PM dev??

Dual-mining command-line examples:

ETH on ethermine.org ETH, Blake2s on Nicehash:
  PhoenixMiner.exe -pool ssl://eu1.ethermine.org:5555 -pool2 ssl://us1.ethermine.org:5555 -wal YourEthWalletAddress.WorkerName -proto 3 -dpool blake2s.eu.nicehash.com:3361 -dwal YourBtcWalletAddress -dcoin blake2s
Nicehash (Ethash + Blake2s):
  PhoenixMiner.exe -pool stratum+tcp://daggerhashimoto.eu.nicehash.com:3353 -wal YourBtcWalletAddress -pass x -proto 4 -stales 0 -dpool blake2s.eu.nicehash.com:3361 -dwal YourBtcWalletAddress -dcoin blake2s