Ownage, the only way you'll know for sure is to experiment.
In my mini-SUV, having the subs on the oposite left/right sides and directly behind the rear seat firing at each other is one of the best positions for them, better yet is both firing straight up from directly behind the rear seat, a little better than that is both woofs kinda' firing toward each other and to the rear at 90* angles of each other like 2 diamond shapes with the woofs on the rearward/inside facing sides, and better than that is each sub in each furthest rear corner firing up.
They are not happy: firing straight forward from the far rear or moved closer to the rear seat firing forward, firing up and back at a shallow angle, firing into the rear corners (tail lights), firing toward each other from the far rear corners, firing away from each other with both in the middle up against the back of the rear seats, and about a dozen other angles ****.
Your individual vehicle defines your sweet-spots/angles/positions. If you build a couple little cubes, you can really go crazy with experimentation. Then, either keep the cubes or build an enclosure that positions the woofers in the way they sounded the best when you were experimenting with trial/error.
Cheers