Has your male ferret been castrated?
One or other animal should be neutered as there is evidence that some females, unless mated, will develop bone degeneration, continuos season and may even die as a result.

As a toileting problem, restrict the male ferret within it's housing to a sleeping space and a litter tray. Try different materials in the litter tray after every toilet the ferret performs in the wrong place. It may just be that he doesn't see the litter provided as suitable one. Sand, soil, wood shavings, sawdust, cat litter, leaf litter or shredded paper could be tried. Try leaving a very thin layer in the tray and a heap of the material in one corner of the tray.

It is highly possible that he his marking territory.