[Boatanchors] 0A2 and 0B2 in Parallel?

[eugene at hertzmail.com]

Hi there. I just acquired a very nice Hallicrafters HA-1 TO Keyer. I was reviewing the circuit diagram from Bama and saw in the power supply, a 0A2 in parallel with a 0B2.  I am assuming that they are paralleled to permit more current to pass, but why not use two of the same?  What is the difference between these two tubes?
 
Thanks in advance
Eugene
 
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