In checking the RCA tube manuals that I have, the 6AQ5 from the 1947 version shows all kinds of caveats with using the 6AQ5 in series heater string configurations. None for parallel.
In the 1960 version of the tube manual, the 6AQ5 is shown as discontinued and replaced by the 6AQ5-A which has been designed with a heater which can easily be used in series configurations.
It seems that this particular tube was a common one in TV’s and seems to be the big reason for its re-designed heater.
Basically the tubes are interchangeable in parallel configurations.
BTW — Fair Radio Sales in Lima, Ohio are showing a listing for 6AQ5 pull outs in a lot of 10 for $10 US.