Hi
I’m taking the SAT tomorrow and my ticket is just my first and last name. I have two last names though. On my IDs it has both last names. Yesterday I called college board and they said to send 2 forms of ID for them to fix it and I called again today and they said that they were still in the process of fixing it. I’m scared that they won’t fix it on time. Would they still let me in if its just my first name and first last name? They match exactly I just don’t have the second last name.
You can explain to the proctor at the door that College Board messed it up and see what s/he says.
Chances are the proctor will be okay with it - If not, call up college board when you go home, demanding a refund and or an immediate make up test as it’s their fault 
^ Good alternative, although good luck on getting the refund. 
The exact same thing happened to me. There’s no space for an extra last name when you register for the SAT. But I had absolutely no problem at the testing center. As long as those names are on your ID and you look like your ID picture it’s going to fine.
I had to contact some colleges (about 3) to tell them about this though since they couldn’t link my scores to my application because I applied to colleges under my full name including two last names. Once I explained this and told them the name I was registered under for the SAT they were able to find it and update my file. So make sure you check your application status when you apply to make sure they’re not missing your scores.
This is probably a common problem, since it seems that some people who design computer systems did not have the foresight to handle the multiple surname case.