Hard to say what's going on......do you have at least 2-3 litter trays in different locaitons? The locations should be in low traffic areas. Are you scooping at least 2 times a day, and replacing with clean litter so that you maintain about 2 inches of litter in each litter tray? Replace litter about every 2 weeks, wash the litter boxes and replace with clean litter. You may try some Dr. Elsey's Cat Attract LItter by sprinkling some on the surface or more of your litter boxes and see if that makes a difference. Cover your beds completely with a sheet of plastic. Hopefully you can get your kitties re-trained to use their boxes. All the best!
Yes and make sure when washing any bedding that you use a product that gets rid of the urine odor. Something like Simple Solution Urine Destroyer which I found at Petsmart. You may not be able to smell anything after washing well but the cats can still smell it and it will draw them back to the spot to do it all again. Is there a particular time of day they are doing it such as when you are not home? Could be separation anxiety? If that’s the case they may need to be put in a large cage when your at work or gone. Or kept in just one room. Make sure you have plenty of liter boxes around for them in various locations. Also have them both checked as one could have a urinary infection and maybe they cannot hold it to get to the liter box.
We also had the issue a few times with our kitten, but she stopped doing it after 2-3 months (so 5-6 months after birth). Directly wash the bedding at high temperature with quality detergent (it contains enzymes to destroy the cat urine molecules). Make sure litter is accessible and plenty available, and clean it every day. Do not punish your cat for the behavior, unless it is directly done whilst peeing. Not after some minutes or hours, she will not understand and will be confused. Good luck.