What Do I Do, I Have 2w2's From Same Employer. Only Boxes 18-20 Are Different On 1. Which One Do I Start With?

Start with the one that has the most information filled in.  

The second W-2 is either reporting additional information for boxes 18-20 or corrected information for boxes 18-20.  Double check the second W-2 to see if it is marked as a corrected W-2, or W-2C.  

If it is a corrected W-2, then replace the information for boxes 18-20 shown on the first W-2 with the information on the W-2C.  Then on the follow-up screen after you have entered the W-2, check the box for corrected W-2.

If it is not a corrected W-2, add a second line of information for boxes 18-20 when you are entering the W-2 into TurboTax.

Either way, you will only have one W-2 entry in TurboTax for that employer.