Say you had a sedan that had a trunk that would fit a box big enough to satisfy four 12" or 15" subs. Would there be any negative effects if you fired one pair directly into the cabin, and the other pair at the back of the trunk?
Bad idea. Speakers playing the same freq range should couple together. But when some are firing rearward, and some forward, it will alter the perceived freq response between the two setups. It would be much like running two different types of subs.