I have a cat who used to do that as well. She would just pee on anything that she saw fit and we think that it may have been a dominance thing over my other cat. We tried a product called feliway. They make a spray or a wall plugin that is supposed to stop a cat from urinating in places other than the litterbox. Here is a link to it on the petsmart website:http://www.petsmart.com/search/index.jsp?kwCatId=&kw=feliway&origkw=feliway&sr=1
And a little information on it:http://www.feliway.com/gb
It basically a synthetic cat pherimone that tricks a cat into thinking their environment is safe and there is no reason to urinate or mark. It really worked well for quite a while, probably about a year, and then she started up again. So we took her to the vet and found out that she suffers from chronic urinary tract infections and we put her on a special food and she is better now. She rarely has an accident anymore.
I think if I was in your shoes I would first take them to a quick vet checkup to make sure there isn't a medical reason for the urinating. If they get the all clear from the vet then I would maybe try the feliway. It's a little on the expensive side, but it's a much better alternative to replacing carpet and furniture and a bottle would last us for a few months.
The vet also told us that it is very important to have one litterbox for each cat. So, with 2 cats you will need 2 litterboxes. This is because each cats like their own place to go to the bathroom that is private. Even if they use each othes boxes you should still have 2 of them.
Here is a good site that has some info and tips to try:http://www.peteducation.com/article.cfm?c=1+2174&aid=158
Hopefully I was able to help a little bit.