Buddhism, getting to its pure form isn't a religion. At the core its an idea, a quest to be 'in tune' with your environment, and a quest to become enlightened - remembering that enlightenment is not a mystical power but acheiving a thought process level/method a bit better than normal (a balanced mind).
In fact, its quite OK from a Buddhists point of view to be Buddhist AND Christian/Muslim/Hindu.
Of course Buddhism has acquired its offshots, its different sects and a bit of mysticism thrown in.
You don't have to do anything special to be a good Buddhist, the goals and rules you live by are acceptable as long as you find them acceptable in your own heart. Its a common misconception that you have to shave your head, beg for rice, and not kill grasshoppers to be Buddhist.
And that is why you can be Buddhist and an Atheist, or Buddhist and Muslim, or Buddhist and Christian.
One of the things I respect most about Buddhism is the monks, most traditional Buddhist (Asian) families send the boys away as monks for a few years. So most Buddhist monks do in fact lead normal lives for a large portion of their lifespan, and most Buddhist men have a crack at being a monk. Beats turning out the kiddie fiddlers that some other religions seem to specialise in.