There will be no need to amend your 2019 tax return if you enter the excess elective deferral amount under Less Common Income -> Miscellaneous income, 1099-A, 1099-C -> Income not already reported on a Form W-2 or Form 1099, answer Yes to "Did you receive any other wages," Answer Yes to "Did you earn any other wages" and select Other. Doing so will cause TurboTax to include the excess deferral as wages on Form 1040 line 1.
If you are doing in this in 2020, you'll have Forms W-2 from both employers near the end of January 2020 showing the amounts of the elective deferrals. If you are doing this before the end of 2019 your proof, if needed, is whatever account statements or pay stubs you have showing your elective deferrals to the other employer's plan.
