WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Some manufacturers provide a switch to bridge a stereo valve amp to a higher powered mono amp. How is that done in a valve amp with two output transformers?
If you must, get two amplifiers with 4 ohm taps and connect them in series (+1 to speaker, -1 to +2, -2 to speaker). Be absolutely sure they aren't grounded together internally!!! (Likely if the amps have NFB.)