If you wanted to build a double ended Fairlie that actually had two independent boilers, you could model one of the Junin Railway's 30 inch gauge 0-6+6-0T Fairlies. Two of them were built in 1905 for the Junin Railway, a nitrate line in northern Chile. They were 40 feet 4 inches overall, and each outside frame power bogie had a six foot wheelbase. For more details and plans, see Donald Binns "The Nitrate Railways" from Trackside Publications. I agree with Roger that independent r/c servos in each bogie should solve the problems of mechanical connections. Of course, if you were mechanically inclined, you might be able to replicate the system the Junin engines used to connect the valve gear with the cab: they had vertical rods on each side of the engine that were connected to a horizontal shaft atop the boiler, a system very similar to the one used on the DSP&P 3 foot gauge Mason Bogies.
If you modelled one in 1/22.5 scale, the Roundhouse 0-6-0 power bogie and boiler should be about the right size, although the smokebox would need to be slightly extended.
Ian Pooley