| What keeps the "other" lamp slightly glowing with the RS ballast is the bleeder capacitor or resistor parallel to the Neutral lamp. Normally it's function is to prevent the ballast OCV from evenly distributing across the series lamps when they are not yet ignited, but rather expose one lamp to the full voltage and once it ignites, it's low drop will overcome the bleeder impedance to expose the other one to the nearly full voltage, so it as well easily.
When you remove the lamp parallel to the bleeder, what you get is essentially a high impedance ballast for the remaining lamp, so in fact powers it with low current, hence the dim light.
Normal HPF RS ballasts use capacitors as the main ballasting impedance. So when you make it switchable, you may alter the actual lamp power.
As RS ballasts provide constant heating power, the cathodes should be fine. They will run slightly cooler, but on the other hand the current is low, so even when restricted, the emission should be sufficient.
The concept of the electronic may be the same, differing only in the operating frequency. The high frequency then makes even low value capacitor passing way higher current (with many designs the capacitor bleeder is nothing else than just a stray capacitance of the interconnect wires to ground) than the 60Hz magnetic, so the remaining lamp glows brighter. Or because even the capacitance between the lamp body and ground is quite significant, you may easily get just partially lit tube, with the intensity gradually decreasing towards the cold end. Many people here tend to misjudge that with mercury starvation or other lamp atmosphere defects. But unlike with the atmosphere defects, here the lamp runs cold and the brightness increases when you touch the lamp. It may glow pink, when the low temperature is not sufficient to evaporate the mercury, so pink just glow is not reliable indicator, whether it is electrical, or chemical problem. For this partial glow, one reason may be missing filament supply and/or cathode failure (so effectively disconnecting the arc), mainly with easy to ignite lamp shape/chemistry (when the ignition voltage is lower than the cold cathode fall).
But back to the dimming: Electronic ballasts may be connected in many different ways, each behaving differently when you remove one lamp, very frequently stressing the internal ballast components. With better quality ballasts the ballast just senses the lamp failure and just shuts down to protect itself.
So generally I would not do that way. It is strongly dependent on the actual ballast topology. If you have some particular ballast, it is way easier to just modify that ballast for the dimming operation, provided the ballast has some means to power the filaments.
With RS ballast it will be an extra capacitor in the lamp circuit (but the filament winding has to remain connected to the filaments; the capacitor works well with both HPF, as well as the NPF shoplight ballasts, you only need to gain access to the internal connections).
With electronic using dedicated ballast IC the simplest way is to increase the operating frequency by changing the timing component (but you will need either a relay or optocoupler for that, as the connections to those elements should remain very short, to not pick up any noise). With the frequency change the typical lamp filament circuits automatically generate filament power (I haven't seen any mains powered HF ballast circuit, which wouldn't do so). The only problem is, you can not go below about 30% brightness without any lamp current monitoring feedback or so, without the circuit becoming too temperamental and/or even unstable.
All the HF dimmable ballasts use the frequency change, they just use some sort of feedback system to monitor the arc in the lamp (via arc current and/or impedance caused phase shift sensing) and are constantly adjusting the frequency so, the lamp brightness remains where it is commanded by the setting.
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