Lots of affection and reassurance helps here, just to let them know that they haven't been forgotten. You may have recently changed your brand of litter, and the cat simply doesn't like it. Perhaps there is deodorant in it, or it is scoopable, and they're used to clay. If this is te case, revert to your former brand, and both of you will be happy. Sometimes you can't determine why they are doing it.
Urination is used as a territorial marking tool by uneutered males, and unspayed females in heat.
If your cats are altered, healthy and not stressed, you may wish to try one of the following. They're not going to go where they eat or sleep. Cats hate the crinkly sound when they walk on it. There is no scientific evidence whatsoever that this is of any benefit to the animal. Many cats can come into heat as early as five months, with males being able to sire kittens at that age as well.