If the entire year's contributions are shown on both W-2s, contact the employer to obtain a correction of one or both of the W-2s so that the total of the code W amounts on the two W-2s equals the total amount contributed during the year. Note that the code W amount includes both contributions made by payroll deduction and separate amounts contributed by the employer.
If they made this mistake with the code W amount, it's quite possible that they also made the same mistake with other box 12 amounts.
