Yes, one pair for 300V dc and ground and one pair for 48V and its own separate, isolated 0V. The heaters are doubled up simply for current rating and minimising voltage drop. I generally use dc heaters from a 12V supply and the EZ Tube Mixer PCB is laid out for 12V heaters. For 2 to 4 modules it is probably not necessary to parallel wires but when a mixer starts to get a little bigger the heater current can become quite large. It is less of a problem with the ECC88 and 12AX7 tubes used in the EZ Tube Mixer ( 4 modules should need less than 2A heater current) but the last mixer I developed used 16 x 6CG7 and 2 x 12BH7 which is over 5 amps of heater current at 12V. The nominal current rating of the cable I mentioned is 2.5A hence the need to double up.
Cheers
Ian
