If the hen that is slacking off laying is behaving normally and not acting sick, my guess is she's still recovering from molt and hasn't recovered her normal weight. Until a hen reaches a minimum weight, her hormones will remain in abeyance and she won't lay.
And then there's the light factor. If anything, the other hen is likely to stop laying at the point where daylight reaches its minimum.
Compare the weights of these two hens. Assuming they're the same breed and age, they should be close to the same weight. If this hen is light, try feeding her some high protein foods as an extra treat. Boiled egg, tofu, cat food, liver, mackerel, etc.
I do this with a few of my hens that seem to have very hard molts and lose a lot of weight.