In Solana, you need to pass in both the keypairs of the signer, and the keypairs of the accounts you're creating.

const {sendAndConfirmTransaction, clusterApiUrl, Connection} = require("@solana/web3.js");

let signer = Keypair.generate();
let anotherKeypair = Keypair.generate();

let transaction = someInstruction(signer, anotherKeypair); 
let connection = new Connection(clusterApiUrl('testnet'));

sendAndConfirmTransaction(
  connection,
  transaction,
  [signer, anotherKeypair] // <-- If you made the keypair, you probably want it here!
);

If you're using a wallet connect library, like @solana/wallet-adapter-react, you don't have the signer, but you will still have any keypairs of accounts your generating:

const { sendTransaction } = useWallet();

const anotherKeypair = Keypair.generate(); 

const signature = await sendTransaction(transaction, connection, {
  signers: [anotherKeypair] // You probably want to pass in the keypair here!
});

This worked for me.

tx.partialSign

let tx = new Transaction();
const anotherKeypair = Keypair.generate(); 

tx.recentBlockhash =  (await connection.getLatestBlockhash("finalized")).blockhash;
tx.feePayer = new PublicKey(publicKeyStringfromWallet);
console.log(tx)

tx.partialSign(anotherKeypair) // THIS SIGNS for web3 keypair

const { signature } = await window.solana.signAndSendTransaction(tx); //THIS signs wallet keypair