Hello again,

Here is one hypothesis, only I don't know your current situation so I'll try
my best. I am not an expert either, I am just going by my own observation
and reading on cat behavior.

It could be territorial. If you had the bed before you got your kitties,
than they could have marked it as theirs. Once a cat marks its territory,
they will continue to mark it from time to time, even with you on it. Your
cat could have peed on you to remind you that the bed belongs to her. If you
got them as kittens they may not have peed on it until they got older, when
they mature and become territorial. Another reason may be that you already
had your kitty and you introduced the bed at a later time. Doing this sparks
their interest to mark it, to makes it theirs. If they have peed on the bed
before the litter tray incident, than peeing on it again made no difference,
only a matter of convenience. Also, if your kitties are not from the same
litter, or were each introduced and raised at different times, than that can
be another problem. You must understand that cats are very territorial.
Eventhough they can live together in your house, it is still divided into
territories. It just depends on which part of the house each one has
claimed. Our cats had a similar problem. They chose a space in the house and
peed on it. They continued to do this until we caught them and kept a close
eye on them. Everytime they tried to do it, we had to discipline them. It
sounds harsh, but it really helps and its for the good of both your
relationships. You don't have to hit them. Like I mentioned before, you can
spray them with water with a small water pistol or try tossing a small,
plush pillow their way as well. Hope this helps. Also the following might.

Our two cats come from the same litter. They usually treat each other well,
Leo the male cat, is very territorial though. When ever Kittie, the female
cat, goes to one of many little beds we have for them, he goes and
intimidates her to get out. Kittie, being so small, really gets intimidated
by Leo who is hairy, weights 16lb and truly people mistake him for a dog.
She almost never puts up a fight with Leo and lets him have all the beds or
areas he wants. Eventhough Leo is big, when Kittie is not in a mood for his
bulling she chases him all over the house, like in a rage. They fight a
little, but after they settle their differences they end up bathing each
other, they have never gone over board with a fight. I guess some cats do
get attached to each other, they groom and they even sleep together.
Sometimes they even eat from the same bowl and supposedly that is rare. In
other cases, there is constant and extreme aggression shown toward a cat by
one of its housemates. This may or may not be associated with defense of
territory (an area that the cat defends as its own). For example, my friend
has two cats, a male named Charlie and a female named Casey. They showed so
much aggression, that most of the time they had to be separated and placed
in different rooms. Cats are very territorial and love to call things their
own, no matter if they're close like my Kittie and Leo or not so close like
my friends cats. Now for jealousy, I do see more of it from Kittie, but I
have found out that it's more like an attention disorder. When me and my
family give attention to Leo, I don't know, but I see some jealousy from
Kittie, unlike Leo who cares less if he gets attention or not, Kittie gets
sad and leaves. The thing is that when I pet them at the same time, Kittie
is just fine and seems happy, even plays with Leo, you know they chase each
other around the house. I guess it's just that Kittie doesn't like to feel
left out of attention.

Regards,
The staff from Prying Eyes