If you want more hog houses, I would try them out as far apart as your garden allows, or where you have suitable sites. We don’t really know how close together is best. Hedgehog Street have been doing a survey, but as far as I know they don’t have results yet. Just be aware that the hogs may decide not to use them. You are lucky they are using the hog house you have – they sometimes prefer to make a completely new nest for themselves, such as under a shed, as Baldwin hedgehog suggests. Just depends how much money you want to spend.
The thing which occurred to me is, if the hoglets are still with their Mother, you may need to keep an eye out for them to make sure they make the minimum weight needed to survive hibernation (450g). Although, if you are feeding them (not mealworms or sunflower hearts) and the weather stays reasonably mild they should have a bit more time yet, to put on a bit of weight.