Wait, is the XR set you're going with the 2 way or 3 way? Either way, you'll need to use all the speakers included in the set unless you want a huge gap in frequency response. As Michael said, just put a single (complete) set of components up front with the W7 pulling up sub bass duties in the rear (or where ever you have the enclosure located). In that tiny a cabin, a single set of adequately powered components should be more than loud enough.
Along with phasing issues Michael/Geo point out, rear speakers that are located a few inches behind your head almost negates the purpose of having quality speakers up front...like the XR's you have your eye on.
