How do I file / print a 2nd return (my sons) on my account? I've already created another account and completed the return (federal and state)

You did his return in a new account?  Or in your account? Online is only good for 1 return per account.  But you can use the same email address for 5 accounts.   How to start another return in the Online version

https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-start-another-return-in-turbotax-online/00/25596

Do you need to find his account?  It would not be in your account.  First LOG OUT of whatever TurboTax account you're logged into right now. Then use this TurboTax account recovery website to get a list of user ID's for an email address. Run the tool against any email addresses you may have used
https://myturbotax.intuit.com/account-recovery/

Then to print...........

Here's an alternate way to print by opening the filed return back up to print at the Print Center, where you can also get the worksheets.

Log in and at the Tax Home or in the section Your Tax Returns & Documents for 2020, look for a link Add a State.  Click on Add a State.  (you don't really add one.  That's just to open your return back up.)

After the return is open, click in the left menu column on TAX TOOLS, then PRINT CENTER.

Then choose Print, save, view this year's return.

The next screen should offer some options:  Just my tax returns or include government worksheets (optional), or include government and TurboTax worksheets (optional.)