Will give it a go. When the walls come thumbling down I'll switch over to 1 transformer / PSU.
If you want some cancellation of the toroid fields, I expect that you would have to wire the primaries in each toroid out of phase. I do not think you can ever expect complete cancellation - just a reduction in magnetic field strength in the regions the two transformers are in physical contact with each other. That said, I have never done this, so I am just thinking a loud here. The stray field from a toroid is not smooth ie uniform flux density - its actually quite lumpy due to wire bunching and uneven winding density even on a good quality component. So, a further step here would be to rotate the cores relative to each other to maximize flux cancelation and therefore coupling into the amplifier electronics, and then rotate the complete assembly with respect with the electronics. Again, I have not tried this, and and just guessing here.
And the central earthing points of the mains are connected with the pre or source earth - I hope so.
The minimum "reverse voltages" of the compensation of the power supply voltages of the connected power supplies then ran over the ground lines of the small signal connections (Cinch, DIO...).