Warning....this might get a little complicated.
Putting two speakers with the same acoustic output next to each other will begin to beam heavily as you get higher in frequency in the direction of the long axis of the speakers. See the attached picture for a representation of this, and image your ear where the red lines meet vs. your ear where the green lines meet. So if you need to use them higher than ~2khz it would be recommended that they sit equidistant from your ear so they sum correctly. Below the point where dispersion narrows they will still sum and act as one large speaker. So if they are going in the back (toward the trunk) of the door panel, put them next to each other horizontally (one closer to the engine, one closer to the trunk). If they are going in the front of the door panel (near the door hinge), you might want to put them at a slight angle (with the front one slightly higher) so that if you measured from the center of one speaker to your ear and did the same for the other the difference would be as small as possible.
Now...will it really make a difference outside of the theoretical implications? Maybe, maybe not. From a purely one axis point of view, dispersion narrowing (another term for beaming) might be good if the speaker has a rising response that needs to be tamed. Also in a car we don't just hear what's coming from the speaker in a straight line toward us, we hear all kinds of reflections. This has a twofold effect though: the combined output of all reflections means that we don't have to be as careful with dispersion issues as you would in a big open room because they will all tend to "flatten each other out", but this won't happen at all frequencies. For those areas you will see a large dip in the frequency response that can't be corrected by EQ, because no matter how much power you throw at the speaker to increase the primary beam output at that specific frequency (or range of frequencies), the environment creates its own reflection that cancels it out.
A good but long thread about this concept if you wish to explore it further: http://www.diyaudio.com/forums/multi...es-matter.html