Could i use one single ledger nano s on both the android app and the windows app using the same balances, or i need to use two separate ledger wallets with completely indipendent balances?
The (hardware) wallet only handles your private keys, your money is still "on the blockchain" (to keep it simple)
So yes, you can use one single Ledger Nano S in both places (if that's also OK for you, depending on how you store your coins).