I found the image below in an old book about vacuum tube rectifiers. Could someone explain why the negative terminal of the load is connected to the 6.3V transformer? I understand why it would be connected to the 350V transformer but not the other one too.
According to the RCA Receiving Tube Manual (30th Edition):
As others have said, this would apply to other tubes in this device, not to the power supply rectifier.
Note the the heater-cathode voltage is rated at -500VDC to +100VDC (DC limitations shown in notes g, h). In this case, it will be running right at the negative voltage rating (-500VDC) since 350V RMS will produce about 500VDC, so the filament needs to be somewhere between the negative rail (call it ground) and +600V.
Also note that this is not two different transformers. As typical for the era, the plate and filament voltages come from two secondary windings on a single transformer. You can tell this by the fact the parallel lines (representing the magnetic laminations) span both secondary windings.