Parallel connection of two heater filaments? To my knowledge, the heater filament in a 6072 tube is only ever connected in one of two ways (parallel is not one of them):
1. Only one half of the twin-triode is operated, and the other is shut down, by means of grounding all elements of the second half, including the second half of the heater filament, which is simple center-tapped to ground. This system is used in ELA M251E, C28A/B, C12, and various copies of these mics. (Curiously, AKG used the second section of the tube, not section 1, for its single-triode models!)
2. Both halves are active, and, aside of the heater filament, are run completely independent from each other. The full-length of the heater filament is used in this case, and its center tap is not connected. This represents what you would call a series heater connection, though it technically is only one uninterrupted heater wire, requiring a doubling of heater voltage to 12.6VDC.
You can either use the second half of the tube as a switchable second gain stage, as it's done in the C12VR/The Tube, or as a completely separate impedance converter/preamp, as it's done in the C24 stereo mic.