Do Tulamps have separate secondry for each lamp or how they work ? Does a lamp with stuck starter or missing affect the other lamp ?

@DieselNut : When you mention you have seen single lamp fixtures with stuck starters for years without overheating, do you mean standard North American potted autotransformer ballasts (like this one) for 30-40 watt lamps or just simple chokes for lamps 20 watt and under ?

I have deliberately put in stuck starters and eol tubes in F15T8 fixtures to test overheating, and the ballast (open China choke) barely heats more than in normal operation. In fact, a rectifying F8T5 lamp in another fixture (with continously glowing starter and mercury migration) made the ballast run a bit cooler than it does with a good lamp. The starter didn't overheat either.

Here on 240v / chokes they can't witstand it forever (and 40W appear to be the worst one in regards to this compared to other wattages), but from few days there won't be damage to it

@sol, yes, I meant the simple open chokes, but the single tube F40 ballasts also do not seem to overheat to the point of destruction either.  There is one under a shelf at a local grocery store that has been running with a stuck starter for at least two years.  Both ends of the tube are still glowing red.  It is a F30T8 and has a Robertson S40B ballast (one tube, 30/40 watt type)

"it still glows" does not mean that there is no damage to the ballast. A ballast can get to very bad shape before it gives up (sometimes with sparks, tripping breakers and blowing the lamp cathodes too)