I've read that nsaid allergy is specific to that one nsaid, but I get hives after both aspirin and motrin. Why? Can I really not take either one?

Unfortunately u cant. But the good news, is if u really need aspirin, and that is a medical decision, you can get it thru a desensitization protocol up to the desired dose, pls refer to an allergist 4 evaluation.
Yes. Many people are allergic to multiple drugs and thus you can be allergic to more than 1 NSAID although this is not common. Unfortunately desensitization would not work for nsaid-induced hives.