FYI
The soon to fly Boeing 767-2C/KC-46 will have both center boom and wing mounted probe and drogue. At least in theory it'll be capable of fueling 3 aircraft at the same time.
BTW, it's my understanding that the center-line boom is far easier on the pilots due to the lack of wingtip vortex that tends to throw the to-be-refueled airplane for a loop (almost literally).