Having them combined together means that the company can more "efficiently" schedule pilots. Efficient means less manning required. It means one group of reserves can be available to fly trips on either plane. It means scheduling conflicts can be fixed by assigning a trip on another plane. etc.
Its a good thing for scheduling flexibility if you're looking for more options, but it is a bad thing from a pilot manning perspective, especially on a long haul widebody fleet.
I think its completely moronic that the FAA is allowing the 777 and 787 to be a common type. Sure, they might fly the same, but they are completely different airplanes. The cockpit is completely different. The systems are completely different. The airplane is completely different. Proof that the FAA is bought and owned by the FAA and the airlines.