Q. Why is my kitten urinating all over the house does it have something to do with being in a different house and using the same litter box with anothcat
A. It could be both of those reasons. You should have at least one litter tray for each cat. Cats can also urinate in the wrong place if they are stressed or anxious which moving to a new house especially with other cats. Try having them in separate parts of the house to start with until the new kitten has settled in a bit, you can also get pheromone diffusers to help with anxiety.