How do I stop female marking her own bed?

Hi,
My female fixed 18mth old ckcs has always marked her fresh bedding - it only now seems to be bothering her "sister" another female fixed bichon. She only marks the bedding in their outdoor night kennel. I have tried to leave them indoors at night but they cry and bark to go out. The kennel has a dog flap on it so being locked in isnt an issue. I have tried to seperate them at night but in the morning they are snuggled up together in the kennel again! Its heartbreaking to see my little bichon sniffing the bed to try and get the least smelly place to sleep on! As soon as i replace the bedding its marked. Sorry if i rambled any help would be great.
She was doing this before being fixed.
thanks

sorry its taken so long for a reply.
Out of interest how do you clean the bedding? Have you washed it in biological washing powder as that should remove the smell.

Thanks, yes, i even tried all different brands of bio powder/liquids, sprays and conditioners!
Thought i had it sussed this week when i gave them two pillows side by side which worked for about 4 nights but today noticed that she had marked one of them (not sure whose it was though)
Back to the drawing board

were they new pillows???

no they were my old ones (which had been washed)

the lazy bones just got up from next to me on the sofa and went to go pee there and then - maybe shes sleepeeing???

Could she have an infection?

Thought that at the start but vet ruled it out, she has been doing this at least a year (shes 18 months now) she was hard to house train and does like to "go" on soft surfaces

Running out of ideas now!
Ummmmm
Have you tried reducing the size of her bedding? If she has enough room to pee and sleep near it, she may not give it up....but if she only has enough room to sleep??? Perhaps put her marked bedding a bit away from her bed...and keep moving it furthur away at the same time as cutting down the size of her bed?

it is really puzzling isnt it, tbh i would at this stage leave her to it only for my little bichon having to sleep in the kennel. I have given her little cushions but she doesnt seem to mind sleeping in pee
i will check in the am if its only the one pillow and at least that way it will be only her own bed.
thanks for all your suggestions

no problem...hows it going?