You need to have 2 devices or 2 different users on same device in order to attest more than one Steem user, so they both would have a separate wallets. And then one can refer the other.

On Windows 10 you can have multiple users on same devices like this https://www.howtogeek.com/226540/how-to-create-a-new-local-user-account-in-windows-10/

What is cool about Byteball is that you could also make joint accounts, these are called multi-device wallets, where both of you have separate accounts, but you also have a joint account, which both of you can spend (or you could also make a wallet where it can be spent only if both sign).