With those amps, the A/B switch simply switches between two sets of identical gain and master controls. There's only one actual preamp channel.
That's the way many channel switching preamps work, especially those that use tubes. Two sets of controls are switched around the amplification path. In this case the tone controls are not switched and are common to both channels.
My Mesa Titan works is a notionally similar way. It has four tubes and two channels that have gain, tone controls, an OD section, and a master volume. Two tubes are always in the circuit. So when you switch from Channel 1 to Channel 2, you are switching the controls, but not the gain path.
The Titan's high gain section is a bit different than the GK. On the Titan, the engaging the OD circuit brings in two additional tubes, plus an OD Gain and OD Level control; so you can set the degree of saturation, and balance the distortion level against the clean level . The two tubes for the OD circuit are shared by both channels as well; so if you switch channels you are really just switching the controls and not the gain path. Basically you get all of the functions of a two channel preamp except one thing...Basically you can't use both channels simultaneously.
Obviously the Titan is a bit more flexible than the MB Fusion since it has two sets of gain controls and each channel can be switched to add a cascaded high-gain section. But it works basically the same as the MB Fusion, and both are considered a two channel switching preamp.
I think the idea is to minimize the amount of components. To make a true two channel preamp, you would need to double the tube count. In the case of the MB Fusion I don't think there is room for six 12AX7s. There's room in the Mesa for eight 12AX7s, but they probably needed to limit the cost as it was supposedly an extremely expensive amp to make already.
I have another channel switching amp as well, my Epifani UL902C. Each channel has only gain and tone controls, as it is not intended for distortion. I don't know if it shares the gain path like the MB Fusion and Titan; I suspect it doesn't since it is solid state. But you can't use both channels simultaneously, so you never know.
