If you have vehicle registration in two states for the tax year, do add them together on one line(or two separate)or can you only use one state?

The total amount of your registration fee is not deductible, just the portion that the state charged you for taxes based on the value of your vehicle. 

In the Car Registration section, when you enter the Vehicle make and model, add the state in parenthesis to show the two state entries.  See image below.
 

Enter your vehicle registration fees:

1. Login and click Take me to my return
2. Next, select Federal Taxes and then Deductions & Credits (if shown, make selections to get to full listing)
3. Click Show more next to Cars and other things you own
4. Next, click Start/Revisit next to Car registration fees.
5. Here you'll be able to enter vehicle description and your fees. Click on Add another row if needed.