The fixture might be wired backwards (positive at conventional negative), incandescent lights do not care but inexpensive solid state electronics
do. I have an old thread where I burned up a USB charging plug
by hooking it to 12v socket that I 'tested' with an incandescent probe....
has both marine-style BA15D for the old school
running lights where the case does nothing and BA15S (like an automotive 1156) for the cabin
lights where the case is negative. I would expect a two filament bulb to be indexed so BA_15D where the space is the indexing type like Y (like an automotive 1157) or U or maybe even Z. I read about something called a SBC-B15 that may have been used in table lamps but nothing there about dual filaments. My RV has one light that dims via the switch and it is just two single
filament bulbs and the switch is off, 1 bulb on, or two bulbs on.
Could you post pictures of the fixture and bulb please?