ok, i'll bite

without trying to critisize by any means, a few thoughts here:
- doubling the output power is close to being un-noticable to the untrained ear, so you may want to make sure beforehand that you gain some gain from the gain.

- a loudspeaker with just 3 dB higher SPL would have the same effect as doubling the output power of the amp, at least loudness-wise, of course not sound-wise
- the power stage of your radio is a single-ended class-A autobias stage. the cap on the cathode of the power-pentode (EL84??) makes for that nice squishy sound when overdriving the amp.
- the output transformer must be capable of handling the doubled DC bias current, when you put another pentode in parallel. that is because you have to reduce the cathode resistor 180 Ohms to 90 Ohms to maintain the DC bias point for double-pentode configuration. the transformer may go into saturation before reaching full (that is, double) output power.
- cathode cap has to be increased to 200 uF to maintain low-frequency cutoff, but that's more a question of taste, so YMMV
- mains transformer must supply the extra anode bias current and heater current of the double pentode. it may overheat in the long term, so you have to keep an eye on it.
- pull all unused tubes from the radio. that will help to keep the mains transformer cool.
- make absolutely sure to
earth the radio, when using it with a guitar. those old mains transformers can develop a hell of a lot of leakage, always good for a nasty surprise!
