It depends on the baby more than anything. My 2 yr old has worn 1 layer Elbee wool cover forever with no leaks. I like them for daytime because they're nice and trim, trimmer than a 2 layer cover. But a baby who is a heavy wetter may have wicking with a 1 layer cover and need a 2 layer, even during the day. My 4 wk old wets so heavily that a 1 layer isn't enough for him, even getting changed very fequently. Also, a 1 layer wouldn't cut it for night time for most babies.<br><br>
Sometimes it dpeends on what kind of wool it is too. A thicker wool, like flannel, may work better as a 1 layer than a thinner wool would.
I think 1 or 2 layers is with wool covers that are made out of fabric. With a soaker which is knit from yarn, it's going to be 1 layer unless you purchase an aristocrat which is machine knit and has 2 layers in the wet zone. As for the covers made out of fabric, interlock is thicker than jersey so you can have 1 layer of it be approximately equivalent to 2 layers of wool jersey. It's all relevant...if you KWIM?