Copper washers are not really what I would consider to be crush washers as in this application they don't have specified torque or measured thickness changes.
If your copper washer really isn't thick enough to do the job (you are annealing it properly before fitting it, aren't you) then I would say a thick washer has got to be better than 2 thin ones, always assuming you don't materially reduce the thread contact.
But having said that, I'd do a detailed comparison of old vs new union, checking thread depth and especially the mating surface where the copper washer sits. Is it properly smooth? Could there be a bit of casting flash or something preventing proper seating of the washer? A freshly annealed copper washer is a vey soft thing indeed, but there's a limit to its ability to conform to irregular surfaces.
