I assume that you ask about doing taxes for another person.
If you use TurboTax online, you'll need to create another TurboTax to file for another person.
Please read the instructions in this TurboTax Help topic.
If you use TurboTax CD/Download, you can prepare an unlimited number of returns and e-file up to 5 federal returns.
Please read this TurboTax Help topic.
Do you mean another W2 or 1099 for yourself? You have to file all your income on the same tax return.
You cannot change or add anything on the return that you e-filed. It is too late, just like when you put an envelope in a US mailbox
Now you have to wait until the IRS either rejects or accepts your return. If your return is rejected, you will be able to go into your account and add or change it and resend it.
If the IRS accepts your return, then you have to wait until it has been fully processed and you have received your refund or paid the tax due. THEN you can prepare an amended tax return 1040X and mail it in. You cannot e-file an amended return. Meanwhile, DO NOT change anything on your return until you start the amended return. If you did change or add the missing info then take it out and put your return back to the way it was when you first filed it.
You can check your efiling status here,
https://turbotax.intuit.com/tax-tools/efile-status-lookup/
How to amend an Online return
https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/00/27577