I would say that it was technically possible for the developers to create special conversations when certain party combinations were used, but that it wasn't a priority — developers are always rushing to try and have a game released on schedule, and customized dialogue for special party combinations may have been deemed to gratuitous at the time. In Chrono Trigger: Crimson Echoes, I am pretty sure that the characters converse often with each other, but this is due to the fact that you are forced to use specific parties for a large portion of the game similar to the beginning parts of the original Chrono Trigger.