K., thank you so much for asking this question. As a mom of preschool twins, I can definitely tell you that kids at that age notice if one person gets a toy and the other doesn't. And they also may like different things. Even though our twins have the same birthday, we have always done two cakes, and made sure that each has at least one present to open from mommy and daddy.
But as for you, the party guest, please get them each their own gift, and you can ask their parents for some ideas. I agree, they didn't get to choose their birth date, and each kid needs the excitement of opening and holding the presents. Whether they decide to share them later on is their choice. One gift each would be great. If you get Ants in the Pants for one, get Hi Ho Cherry O for the other. Or whatever - cars for one, trains for another, etc.
On the flip side, when my twins get invited to another child's party, we bring a gift FROM each of them as well, because not only are they not the same child, they like to be able to give a gift or bag to the child themselves.
Oops, I meant to say too, bath ideas are great if they still share. There are a lot of cool bath things, entire playsets, and some that stick to the wall, etc. Get each of them one bath toy and if they want to swap out in the tub they can!