Single vs Dual Power Transformers

The issue is power supply modulation, which can be very difficult to prevent! If the supply has noise (IOW, if the action of the amplifier has caused fluctuations in the power supply) the result can be intermodulations with the other channel running off the same power transformer.

This is why monoblocks usually sound better than dual mono, and why dual mono usually sounds better than a plain stereo amp; intermodulation distortion is pretty audible.

If a single power transformer is used, are separate secondary windings for each channel used in all stereo amplifiers or is this configuration in the dual mono or partially dual mono camp? I'm just trying to understand why some manufacturers hype separate secondary windings as complete power supply separation for each channel, despite a solitary power transformer.

It is better to have dual windings than to have both channels share the same supply. The weakness of this approach is that the two supplies still can 'talk' to each other through the core and primary winding of the transformer, although certainly there will be less noise than if the two channels shared the same supply. IOW this is a step to better performance; better yet would be dual power transformers.

There are a lot of variables at play here so nothing can be considered case in concrete; I am merely stating how this works strictly from the point of view of power transformers. There can be other considerations in a design that may negate this issue- ultimately audition is the best course of action.