If you are already showing a refund but need to enter a 2nd w2 but dont have it, what should i do?

You have options:

You can request a W-2 from your employer, enter the data, and then file your return,

OR

You can file your return, request a copy of the W-2, enter the additional information, and then file an amended return,

OR

you can enter the details on your employer from a previous year's W-2 and download the W-2 following the instructions provided by TurboTax. 

You also may be able to request a Wage and Income Transcript online from the IRS which will have the federal information from all your W2s.  You can use this link for the request. The transcript will not have state information so the best source is your employer.