My son was invited to a classmate's birthday. This little boy
is a twin. My son doesn't know the twin.

On the birthday party invitation, it said, "You are invited
to Bob and Rob's birthday party!!" (These are made-up names.)

We only know Bob. DS and Bob are in class together.

We do not know Rob. Rob is in a different class.

So, do we buy both a birthday gift? Or do we only buy a birthday
gift for the twin that we know?