My experience with the WH from Holderread's was not that at all. Other than that they did have a period of time where they "met in the middle" and both looked the same. My girl's bills all changed much faster than the boys. They began getting darker literally when only a few days old (which is what is supposed to happen). A spotted bill at several months old strongly suggests to me that the bird is not carrying the sex-linked bill color (and I don't mean that as anything against Metzer's because I like them a lot and think they have good, healthy birds). Anyway, my girls had very dark bills within a couple of weeks.
The boy's bills began to lighten more slowly over time and were the exact color of the OP's dark headed duckling until they were about 6 weeks old. They continued to lighten and get more green over several weeks/months. I definitely agree there was a period of time where they became difficult to discern by bill color. The difference was so slight, but it was there. I would say the OP's are most likely a pair, but they could be two females (I'm strongly leaning towards pair though).