Yes and no. No because the tremolo works like an automatic master volume control on the power amp. If you substitute the two 330K grid resistors on the 6V6 tubes for two pots (or better a dual pot) you'll have the same effect, but manually activated. The tremolo oscillator makes the volume to change automatically. That's all.
...and yes, because the new channel affects the sound even if a guitar is plugged to the Twen channel only. But this happens in any amp mixing its channels with resistive mixer like this. Virtually any old amp without true independent and fully switchable channels. Some designs are very interactive and other doesn't interact that much. The most typical example of this is the Neil Young trick I mentioned before. The basis of his sound is a Tweed Deluxe combo, an amp with hugely interactive channels. The idea is plugging the guitar to one of the channels and set the controls to get a sound. Then by actioning the volume and tone (mainly the volume) of the other channel we can get the amp to sound juicer, or even crazily distorted as it works in these Deluxes. Young uses an automation system having four electric engines moving the four pots in the amp, controlled by a four presets pedal board. This way he can go from pristine clean to his typical crazy overdrive with two steps in between in an instant at the flick of one of the four switches.
Just try this effect in any old amp with two channels. Turn the non used channel volume pot and your sound will change. Usually the sweet spot for the max gain added by the second channel is about 2 or 3 O'clock. There are amps with reversed phase channels, like the blackface Fenders with reverb or the Vox AC30 top boost, where this effect might be detrimental due to phase cancellations, but it's just a matter of taste. I don't like it, though some other guys love it, as they would love their pickups out of phase on a guitar too. Not for me.
Anyone using this mod? No idea. I conceived it a while back but never told anyone. It was just an idea I wanted to try and never or put it to work. But it's nothing crazy. Sure anyone out there has an amp close to this. In fact, the Fender 5E3 Deluxe or the 1482 have essentially the same basic topology. This was the idea behind this mod.
If you want the original Twen sound unaltered just add a DPST switch to disconnect the mixer and the 1482 channel. (And don't change the 330K plate resistor on the second stage)
Oooops! I said:
If you substitute the two 330K grid resistors on the 6V6 tubes
They are two 100K resistors tied to a 200K tied to ground. This makes the tube grids to see 300K each. The 1482 has 330K resistors instead. About the same. You can leave them in place the way they are arranged, just removing the tremolo connection from them.