Wood floors can be worse than carpeting. The only house I ever had fleas in had wood floors. But it was an old house and the slats weren't 100% tight anymore, and that's where the fleas lived. I actually had to stay with a friend while I treated the cats, the yard, and the house. And then had to do it again a week later, to kill all the fresh-hatched ones.

If one of your cats has fleas and the other doesn't, make sure he doesn't have tapeworms. Zebo had fleas but Toot didn't. However, Toot was a nervous cat and frequently overgroomed. He actually did have fleas, but groomed so compulsively that they didn't last long enough to be seen before he was swallowing them - as evidenced by the galloping case of tapeworm he had!