Dale, the situation is indeed complicated when connecting different power sources to adjacent blocks.
(First, let me clarify that "dual roller" cars are a problem ONLY IF the rollers are connected together internally. Many passenger cars have two rollers, but each roller is connected to its own isolated set of lights, with not interconnect between the two rollers. I recommend inserting a "Polyfuse" self-resetting fuse in series with one of the roller wires to provide protection in the event of a derailment that places one roller in contact with the center rail and the other roller in contact with the outer rail.)
The "added complication" is that the nature of the power controller needs to be factored in. Some of our controllers, like the Powermaster for example, will supply power, but will not sink power - somewhat like having a diode in series with the output that reverses its polarity on each half cycle of the AC waveform. Power from another device of a higher voltage connected across the output of the Powermaster cannot flow backward through the Powermaster.
The Powermaster uses two FETs in series, with one being the controlling device and the other acting as the diode. The roles of the two devices reverse on each half cycle.
The TPC uses a single device that is essentially a Triac. When a Triac is turned on, it can conduct current in either direction eually well. Although I haven't specifically tested this (disclaimer!!), this would seem to allow a TPC to sink current back to its transformer source if a higher-voltage source is connected to the TPC output.
I don't know what devices are used in the newer ZWs or MTH products.
If the controller device has a blocking "diode", then dissimilar waveforms don't create a big problem. The higher-voltage supply may temporarily try to power both blocks while any dual-roller units with internal interconnects bridge the blocks, but there won't be any power fed back into the other transformer.
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