After Sec. Dr. Gates' recent remarks against KC-X split buy, Rep. Norm Dicks of Washington has re-affirmed his opposition as well. Both oppose split buy because they believe operating two aircraft types will be expensive to the USAF.
Dr. Loren Thompson made a case that splitting KC-X may be fiscally advantageous to the USAF, based on some budgeting considerations. However, he failed to consider the engineering angle of lowering operating expenses. Which makes sense, since he is not an engineer. I am, though, so I will make the case that split buy can work, in terms of maintenance expenses.
One important point you need to remember about Boeing KC-767 and Airbus KC-45 is that they both can use the GE CF6-80 series turbofan engine. The two platforms use different variants of the CF6-80, due to different thrust requirements. All CF6-80 variants share common turbine stages, however. Therefore, in terms of engine spare parts, KC-767 and KC-45 share many common parts. Engine maintenance/rebuild accounts for the majority of aircraft maintenance expenses. On the engine side, a split purchase does not much increase the maintenance budget due to the volume scale.
Another point that critics of split buy bring up is the requirement for separate training pipelines. To non-pilots and non-engineers, that excuse sounds valid. That is not the case. We can build the two platforms to maximize training commonality. For example, a big part of learning to fly is the "knob-ology", learning to interpret the displays and gauges, and to operate the knobs and switches. The reason there are different training pipelines is because different planes have their gauges at different places. Whereas you do not need "training pipelines" for GM vs Toyota cars because they all have their tachometers (engine RPM) right next to the speedometer. Cars can share the same "training pipeline" because they share the same "knob-ology" with minor differences. 767 and Airbus A330 currently have different flight decks, necessitating the different training programs. If the USAF were to mandate a common cockpit for the two platforms, this will obviate much of the need for training pipelines. The A330 uses fly-by-wire, so we can further restrict its flight envelope to mimic 767's flight characteristics, if necessary.
As you can see, smart engineering can minimize the expenses of a split buy over winner-take-all. If the USAF spends a little effort in its system engineering and acquisition, by mandating the above two, common engine and common cockpit, it can make split buy work. Of course, this route requires either commitment from senior USAF leadership or a Congressional mandate. Congress will probably need to cram this down USAF's throat.
[If you're going to say something about the unfairness of GE monopoly, well, we can have a "split-buy" there, too. Just like what we're doing with F-135 and F-136. A common engine-aircraft interface is all we need to get that done.]
Update 2MAY2009: Dr. Rebecca Grant of Mitchell Institute/Lexington Institute has released a paper, The Tanker Imperative, which echoes the arguments made by Dr. Thompson above. Still no mention of the engineering argument I made, tho. Hat tip: The DEW Line.