How old are your kitties? If they are males that have just now become sexually mature, that could be a cause for this sudden change. (especially if they are BOTH doing it)

Next step is to think of anything that might have changed: Type of litter, placement or type of litter box, new furniture or new layout of furniture, are the boxes dirty or is the level of the litter too low?, new person in the household?, new animal in the household?, stray cats outside that your cats sense?, anything?!

And remember, a lot of times it is a medical condition so make sure you have themn checked.

Wal-mart has an enzyme cleaner in the pet section called OUT and I have used that with good results. But with any enzymatic cleaner you MUST SOAK the area, do not skimp.