why is someone peeing on the dog beds

Dogs don't "get back at" people for things or pee out of spite.  I would guess it's one of three things: 1) someone needs a refresher course in house training, 2) someone has a UTI and should see a vet, or 3) someone has now developed a habit of peeing there and likes that texture, smell, etc (see also #1). 

I agree with Liesje. If you're sure there's no UTI, I would take out all the beds for a while so the habit gets broken OR put the dogs in crates during the night (basically re-house train) The problem with taking the beds out is that the carpet is the next best thing and the problem will just transfer to there. You could separate them with a baby gate or a crate to determine who is doing the deed.

I wouldn't make any assumptions about who it is until you know, but I don't think dogs use peeing to get back at anyone.

Good luck!  

 How old are your dogs?  If they are older, then it may be some incontinence.  Also, have either of them begun to drink noticeably more water lately?  This may be a symptom of diabetes in which case a vet trip is in order.

I would second the seperating and/or crating to find out who is doing it so that you can be sure to find out why. 

Dogs don't "get back" but they do mark their territory and things with urine.  I watched my elderly dobe mix (who was definitely the dowager queen) walk over and pee on my setter's dog bed.  My setter was a bratty youngster and never showed her the respect that she felt she was due. The dobe would work hard to confiscate everything she could from the setter.  My pointer is very socially adept and did all the doggy right things to show respect and was actually allowed to lay next to her and touch her paw. 

This wasn't "oooo oooo, I have to pee because I have a UTI" and it wasn't "I really like to pee on dog beds now".  It was definitely meant to be a deliberate statement to Trudy ("This is my domain, not yours";).  She was looking straight and hard right at Trudy when she did it. When she went for a walk, she would sniff and mark places like a male dog, too.  

I washed all the dog beds.  I have the kind that can be dumped into the washer with OXI and hot water and washed thoroughly.   I made it clear that the dobe had the place of honor (next to the door) and Trudy's bed was put on the back wall where the Dobe never slept.  Any more squabbling about those dog bed and both beds would be on the foot of my bed where no one is allowed.  After all, they are ultimately MY beds.

dogs do mark THE BOUNDARIES of their territory with pee, but no dog with intact instincts pees in the "'living area". dogs peeing on beds is either a housebreaking issue (dogs naturally pee on absorbant materials, i.e. beds and carpets) or a medical issue- must pee oh look that's not my bed but it's absorbent. A dog who can't hold it due to a medical issue sounds more likely in your situation. Dogs do not pee on objects to "demonstrate ownership" or "demonstrate dominance" or to "spite you".

Nope it wasn't medical.  She was not incontinent or having a lapse in her house breaking.  The very few times she had accidents in the house (usually after a huge dose of prednisone) she was very embarrassed about it. 

She was not being spiteful, she was just marking the bed.  Trudy might have left a bit of urine on her bed and Socks, my dobe mix, urinated on it. She would also quickly run and urinate over the other dogs' urine when she was out side in the yard to reclaim that particular spot.   She was very busy with two other younger dogs to keep under control. lol It wasn't huge amounts of pee.  It was marking. 

She didn't have a UTI - this dog was a hemolytic anemia survivor and was constantly monitored.

So I respectfully disagree.  I'm not going to argue with you but I knew my dog.