Sounds like the bulbs are 9 Volt bulbs. The math would work out if each bulb was a 9 Volt 50 ma bulb and you had one 30 ohm resistor in series with both bulbs. R = (12 volts - 9 volts) / (0.050 amps x 2 bulbs) or R = 3 volt delta / 0.100 amps or R = 30 ohms. Just make sure the decoder function output current rating is not exceeded. If this is a large scale decoder, then nothing to worry about since the function outputs on those can handle loads in the order of amps, not milliamps, but if it is a small scale decoder retrofit, like an HO decoder, then I would double check the maximum function output current on the aftermarket decoder specs as many smaller scale decoders have a 100 ma max current capability so that little mosfet transistor will be running right at the limit, which may shorten its life to the point of becoming a slo-blow fuse 
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Would be curious to know (maybe it is too late and the bulbs have been reinstalled) if you touched the leads of the bulbs to just a simple 9 Volt battery terminals if they would burn at the same brilliance as what you have using your resistors to confirm the bulb's rated voltage.
Scott Kay
Austin, TX
