She is probably toileting in the house because she has had a 'territorial incident' outside.
Cats are territorial animals, and when a cat thinks it has issued with another cat appearing on its territory, it will usually make its superiority felt - either with a warning, a chase-off, or a fight.
If your cat has been on the receiving end of one of these responses, her natural reaction is to underline her own territorial area, to make herself feel more securer, and as a warning to any cat who may feel like taking her on, on her own home turf.
The upshot of this is, as you have discoverd, marking her immediate territory with her urine and faeces.
It is probable that once she has forgotten the incident, she will settle down and return to her outdoor toiletting. In the mean time, if she uses the liotter tray, show her what she has done, and make a huge fuss of her, so she associates using the tray with approvcal from her pack leader - you.
If your other cat habitually toilets outside, he is unliely to bother with the ltter tray.
Hope all this helps.