Two amazing things--people who don't know how to use SEARCH, and others who know NOTHING of which they speak.
RW, inverting inputs and wiring outputs like that is called bridging. I've never known anyone to try it with tubed amps. PARALLELing channels of tubed amps is an old and effective practice, of which I've written many times. The power simply adds--your 100-Watt channels become a 200-Watt amp--into HALF the indicated impedance. If you used the 8-Ohm taps, you now have 200 Watts into 4 Ohms.
To wire it, use a splitter cable to send the identical signal to both inputs and a short chunk of speakercable to combine in parallel the outputs--positive to positive and neutral to neutral.
I've done this with the 2 channels of c-j MV75s and with EAR monoamps. It's a VERY effective way to drive low-impedance loads with tubed amps.
And before someone writes that the power can't be greater because the Voltage is the same as in one channel, just use the next-higher-impedance taps. But even if there aren't, say, 16-Ohm taps to use, the paralleled taps have half the circuits'/transformers' output impedance, so the damping factor is doubled. Then the paralleled 8-Ohm channels are better able to drive that '8-Ohm' speaker that probably dips significantly below 8 Ohms.
Try it, Red, and you also might try SEARCHing next time. :-)