How should I file a 2023 1040 form, which did not get electronically sent, along with a 2024 1040 form that is ready to be sent?

You will have to prepare a new 2023 tax return if it was not on a Desktop Software.

• You can then file both the 2023 and the 2024 tax returns,
• Use this TurboTax cite to File your 2023 taxes

@CV21 

BUT, to be perfectly clear...you DO NOT send them in together.

You can go ahead and e-file your 2024 tax return now....if you have it done.

BUT

You must prepare, print and mail-file your 2023 return by mail..yourself using the 2023 desktop software.   No rush, just get it done by summer.

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Since you didn't file your 2023 tax return on-time, when you attempt to e-file your 2024 tax return, you might need to indicate that the 2023 AGI was zero....during the filing phase.  That's just what the IRS's accepting computer has for your 2023 tax return...since it wasn't filed on-time.