The two species look virtually the same. Only with a magnifying microscope, it is possible to see the subtle differences between the two. Both graze on the microscopic fungi found on dead plant material during natural degradation.
The common house dust mite lives, among other places, in bird nests. It can form blooms in a damp, mouldy room, and it is usually the one you see in newly built houses that are heated, but have not yet dried properly. The sugar mite is on the other hand tolerant to cold environments and is therefore free-living. It is also the sugar mite that dominates in unheated storages containing hay, straw, grain, etc. If the room is slightly heated, as is the case in barns, it is possible to find both species together.