This is true for couples as well as singles. You can arrive at the main dining room and ask to be seated with another couple, a table for 6, or even a table for 8. However, in the specialties, you will not be able to ask for a table for 4 unless you provide the other two people.
I see this as a major drawback of eating in Prime C or Aqualina. When we stay in a suite, we hesitate to ask another couple to join us for dinner because it may be an unanticipated expense for them.