It sounds like everything was duplicated on your two W-2's except Federal Tax withheld. I would agree with you that you should enter the W-2 from the insurance company, showing taxable payments and tax withheld. If your employer W-2 has a Box 18 amount and the insurance company W-2 does not, add that amount to your entry.
However, the issue with the IRS is that if you have a W-2 with an amount in Box 1, they expect to see that reported on your tax return (they do document matching). So, you should definitely contact your employer and relate what the insurance company told you that they were the correct issuer of your W-2. Be sure to document your conversation with them (date, time, who spoken to, etc.) and how/when they said they would resolve your issue. If possible, get a Case number. Save documentation of your contact with the insurance company, too.
If they agree to send you a 'Corrected W-2' you could enter the insurance company W-2 in TurboTax. From what you say, your corrected Employer W-2 should only contain a check in Box 13, with a notation in Box 14, and an amount. This would be saved for your records, and would not affect your tax return, and would not need to be entered in TurboTax.
You could wait until you receive the Corrected W-2 to file, though, so the IRS is not looking for your other W-2 if they receive your return before the Corrected W-2 is issued; however, it's up to you if you decide to file with one W-2 since you have documentation for your situation.
Here's more info on Multiple W-2's (though not exactly the same situation as yours).
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