New kittens

Question
Hi we have just got 2 new kittens both sisters at 6 weeks of age... They are eating dry food great and are very good besides there play fighting that we have been breaking up..
The one kitten has been litter fully trained or at least fine with it...  the other kitten is pooing in it but not peeing for some reason, we have been putting her in it randomly
maybe not regularly .. she was peeing on a pile of clothes and we removed it, now we found on the bed she has been peeing needless to say we would love to correct this as she will only be turning 8 weeks in 2 days now and the sooner we figure the better.
They have been vet checked 2-3 days after we got them
dewormed first shot and flea treated (we would have asked the vet but she was too new to notice yet at that point)
Any suggestions
I was told it definitely is not a territorial thing as they are so young...

Answer
Hi Lisa - get another litter box for her - sometimes they like to rotate boxes.  Give them two boxes and see if this helps.

I have many cat clients with 2 cats and 2-3 boxes.  I know it sounds odd but it works.
Also, you may want to put a different litter in one box.  Two boxes and two litters.
Don't let the kittens have run of the house - keep them confined if possible for a few weeks.

Also, please get them both on NuVET Feline Formula - you will mix this into the food 1 x per day - this is an immune system booster and many breeders send kittens home on this:

Here is the ordering link-
http://www.apluspetcare.com/untitled4.html