Disclaimer: I think this is how you do it but If I'm wrong please correct me.
Firstly, when you create a new wallet its gets stored in the directory where you told it to store it. The default location for me was /Users/COMPUTERNAME/Monero/wallets. All of your .key files will be stored there for each wallet.
Once you have created your first wallet and are logged in log out. Select create new wallet and create your 2nd wallet. Once you have both your wallets created you can log out again.
Now if you want to log into first wallet simple click "open wallet from file" and select your desired wallet. If you left your wallet locations in the default location then all of your wallets should pop up. Simple select your desired wallet and input the correct password to open that Wallet.
\*Make sure you write down Mnemonic Seed and Wallet Restore Height for each wallet incase you forget or loose your password so you can restore your wallet.
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