(I'll post the plans up again for those who want to see how this project was figured out, originally posted in this thread in the General Discussion forum:)
Here are the results. I figured out volumes and measurements for this project (may do this some day myself...) and I will outline it here. All dimensions here are OD (outside dimensions). The two boxes have the same common height at 15", the same common width at 36", and have a common base height of 3 3/4". The face angle of each is 45 degrees, and saw angle cuts are listed here for the angles. The face angle cuts are 45 degrees, but the angle on the rear of the front box has angle cuts of 16 and 74 degrees. The lengths of the hypotenuses are 14 7/8" for each face and 10 7/8" for the backward angle. There is a center top flat depth of 2 3/4" on the front box (I figured the air space directly below it at 1 1/2", so let the final size be determined when you assemble the angled sides), and a rear top flat depth of 3 1/4" on the rear box. The depth of the front box is 16 1/2"; the depth of the rear box is 14 1/2". The two boxes combined make 31" depth.
Volumes were determined using 3/4" MDF. A solid 3/4" MDF center baffle is placed dead center of each box to separate the subs. I figured subwoofer displacement ( or lack of, in the rear box) with these measurements. Each sub will have 1.2 cubic feet of air space (displacement already figured in). Build the boxes exactly to these specs and you will be good to go...(but check for fit in your car first). Just an inch different somewhere will throw the balance off....and in your case where you are powering all subs with one amp channel, we want the volumes to be the same for each sub.
In this pic, the subs have been resized to be truer to scale and the base has been raised. But I didn't redraw the boxes entirely, so the rear box will actually extend forward a bit more than is shown, and the front box will be somewhat compacted with a steeper backside angle than is shown (but the "cut angles" listed are correct)...
Using the same side view, the interior space is divided into rectangles and wedges and calculated to get total volume for each box. Measurements below the pic are interior, and the results are listed below. 1.0766 1.0766 .307 .5127
.307 .76
___.879___ _________
2.569 ft^3 2.35 ft^3
- ___.16___ sub displacement +__.04___ cone area
2.41 ft^3 2.39 ft^3
The middle divider and all sides have already been deducted, so you just divide these box volume totals by two to get 1.2 ft^3 per sub.
Calculators used:
https://www.the12volt.com/caraudio/boxcalcs.asp#wed
https://www.the12volt.com/caraudio/boxcalcs.asp#rec
http://www.1728.com/pythgorn.htm
Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.