Parallel / series heater, only using 1 part of the heater ?

Got here a tube amp with 2 tubes of type 2P1P (russian, datasheet attached).

There's a heater supply line which connects to pin 7 of both tubes. Pin 5 of both tubes goes to ground via an inductor. Pin 1, the other heater pin is left not-connected. If understood correctly, this means that half of the heater is left unused. Is this correct ? Like powering a 12ax7 only using pin 9 and 5.
If so, what voltage and heater current should I use for proper heating of these tubes?

Normally in series (according to the attached datasheet) U = 2.4V and I = 60mA, or U = 1.2V and I = 120mA.
However, when turning up the power supply until a current of 120mA total (for both tubes) is measured, 60mA per tube, I measure 1.50V over the heater voltage per tube, thats before the inductor to ground. Due to the inductors to ground, total voltage is 5,75V but that's of no concern.

This is no circuit of my own, this is an existing circuit, which has been used. 

It might be that the heater resistance has a wide tolerance.
As long as the amplifier works fine with 1.2 Volts / 50 mA (or so), I would not worry too much...
Raising the heater voltage to 1.5 V. will reduce the lifetime of the tube.
(A lower heater voltage might only reduce the emission a bit.)

Bedankt Ruud!

What I'm a bit worried about, with using only half the heater, will the tube actually reach it's necessary temperature inside, or will I have to raise the voltage a bit in order to get it to the proper temperature? And of course, how will I know?

If the power supply can deliver enough current, I would prefer to use both filaments.
Be it parallel (1.2 V / ~100 mA) or in series (2.4 V /  ~50 mA).
In this case the cathode emission will be good and you won't have to worry about the somewhat lower current consumption than specified by the manufacturer.
I would never increase the filament voltage to a value higher than 1.2 V. (Per filament)
Filaments in those tubes are thin and fragile.

Yes, of course. But the circuit originally uses only one part of the filament.

Is there anybody with experience on other circuits, how do the tubes behave under these conditions? I suspect they don't live up to their datasheet specifications?