Does it mean I now have two wallets, including sn?
Yes, you have now two wallets with separate balances: the balance in your SN wallet and the balance in your Mutiny wallet. When you zap or pay for something on SN, we try to pay from your SN wallet first since that always has zero costs and latency. If you don't have enough funds in your SN wallet, we fallback to your Mutiny wallet.
Where do I setup auto withdraw/fund?
NWC currently only works for funding/spending so you basically already setup auto funding. You can setup auto withdrawals with the receive only methods (LND or lighting address for now):
receive onlymethods (LND or lighting address for now):