My 2 employers both withhold FICA but use one payroll service so i over withheld, how do i get credit on taxes paid. Do i need to complete 843, and why not on the 1040

If you received two W-2s, do the two employers have the same EIN?

No the payroll service was the same for both employers and used one W2, thus the over payment.

So, one W-2 that reports more than $7960.80 of social security withholding?  If so, you either have to obtain a refund from one of the employers, or use Form 843.  

You can only get a credit on your tax return if the overpayment results from multiple W-2s with different EINs.