All, thanks for all the good info -- think I will be ordering some Warren two dot rear sights to try.
It seems there are different situations, generally within the category of dark, which are very different. Such as:
1) Shooting with a WML, as I did last night in pitch dark conditions, doing Dot Torture. While my front sight had tritium, using a Surefire X300 on a relatively close target, I don't recall seeing the tritium dot -- just a perfectly defined black front sight.
2) Going from bright to dark, where your eyes are not adjusted. This is where I have a particular problem with one dim rear dot, and prefer two dots.
3) Eyes completely adapted in pitch dark, where the single rear dot might be bright enough, but questions remain, raised by TLG, on the relative precision compared to two rear dots.
Probably a number of other situations I haven't contemplated.