I'm a little confused with your question . . . are you asking about a pair of guns each having two sets of barrels fitted to them? With the frames, forends and barrels all serial numbered identically. (2 frames and 4 sets of barrels). --Or-- are you asking about about a 2-bbl set? (one frame with 2 sets of barrels, with the barrels only marked A and B.
If you're asking about the former, it would be extremely unusual and the first time I've ever heard of it, nor do I recall that scenario ever being discussed here. Furthermore, pairs to most would have consecutive serial numbers and usually be marked with 1 and 2 -- not A and B. But, then again with all things Parker . . . we never say never.
If you're asking about the latter, I suppose it's very possible the barrels only could be marked A and B, but that, too; would be unusual as they are customarily stamped 1 an 2.
Parker Reproduction Driven Bird Pairs were discussed here:
http://parkerguns.org/forums/showthread.php?t=6389
Threaded Mode