You're welcome,
Yes, exactly. The ballast label means for a 40W tube.
If you were to put two ballasts in series, you would be putting two inductors in series, so the inductance adds. It's difficult to think of in terms of watts (the discharge voltages of different sized tubes is different), but hypothetically you would end up with a lamp wattage of less than 50W because the inductance of the two ballasts in series is higher than the 50W one on its own.
Sorry, I shouldn't have put the bit about the "aka PTCs" it doesn't translate well. I was meaning putting an actual mains filament lamp in series. Because the resistance of the filament increases with temperature (the PTC effect), a lamp in series with the supply can be used as a power [Edit: well, current] limiter when testing mains equipment. If the current consumption is ok, the filament remains cool, so low resistance. If, worst case, the equipment has a short, then the lamp will light to full brightness and limit the current to a safe value (hence the common name of 'dim bulb tester'). Because the thermal mass of the filament is very small, operation is fast.
You can arrange a switched bank of different wattage lamps, or just have a single holder and (safely) swap the bulbs, depending on how much you're going to use it.