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.
Doubling the tubes will lower the source impedance (not that it will matter much) but the gain and swing are unlikely to change. If you're using the tubes to drive a phase splitter, the source impedance advantage will be practically nonexistent.