[G6Tanuki]

I guess if the magnetisations from the currents for the single EL84 and the PA valve flowing through the modulation transformer were additive it could have caused core saturation.

Reversing the phase of one winding would have the magnetisations cancel each other out and so reduce the risk of saturation. That's the cunning thing about the Codar-type autotransformer approach, which allows the use of a seemingly-very-small transformer.