Thank you!
Thank you again. That's exactly what I said, if you have two AC conductors that are delivering power and they are the same phase, voltage and frequency, you can parallel them, because there is no potential difference between them. And what you have coming into a house with 240/120 service looks like two 120V voltage sources that are out of phase with each other by 180 deg, or of opposite polarity, same thing. It actually is that. By viewing it as that, all is explained:
A - With respect to the neutral, you see two 120V voltage waveforms with respect to the neutral, differing in phase by 180. And that's exactly what is there, each half of the secondary winding is a voltage source and they are connected with opposite polarities with respect to the neutral.
B - You can't randomly parallel any two 120V receptacles in the house because some are on opposite phases.
If you measure between the two phases, you get 240V.
All that flows directly. There is no need to talk about transformers, generators, or a synthesizer, if it happens to be created by an electronic UPS type device, etc. If you add in ampacity, that voltage, phase, freq info is all you need to design with it and use it.