Can a person live by the Dhamma and have another religion as well? As Buddhism is growing in the west, we see this "conflict" a lot. In Asia I met some Buddhists-Hindus, Buddha, Shiva and Ganesh were equally important to them. Here in the west there are some Christian-Buddhists, people who believe in the Dhamma, but cannot stop Praying to Jesus. Let me add here that according to the Bible if you stop believing in God, you will go to hell no matter what, it is a serious offense for Christians.

I live in an almost 99% Christian country. You only find churches here, you don't find stuppas, temples or monks. It also contributes to the "Christian-Buddhist" phenomena as there is no strong Sangha.

According to the Buddha's teachings you should give up rituals, believe in the Buddha's enlightenment and follow the Dhamma, but on the other hand many people are claiming the Dhamma is more a "way of living" than a religion, so it could be adapted to another friendly religion. 

Can a person follow the Dhamma and be a Christian at the same time?
In short: no, you can't. Even cursory knowledge of the teachings of the Buddha and the dogmas of Christianity (or any other religion) will prove enough to realize these are irreconcilable. Just to give a quick example, take the Buddhist belief (one of the "Four Seals") that all things/phenomena are empty and devoid of self: this is enough to negate the whole of Christian dogma at once! There is no doubt that the view that Buddhism can be seen as simply a "way of life" stems from a profound ignorance of the teachings and practice of Buddhism, usually based on a feel-good, watered down version of the teachings that doesn't even come close to trying to get to grips with understanding something as essential as impermanence, interdependence, and emptiness.

EDIT: The above comment is not meant in any way to insinuate that non-Buddhists cannot greatly benefit from studying the dharma—far from it, especially when most Western Buddhists come from a (at least vaguely) Christian upbringing. It was only meant to express that truly abiding by the Buddha's teachings entails a worldview that is not fully compatible with that of the major religions.