I could make centering rings out of 1/8" ply, I have a drill press, band saw, and wood lathe. But it really depends on the inside diameter of the nonstandard body tube. The outside diameter of a 24mm motor tube is .950" or 24.13mm. The body tube is probably 2" OD. If the wall thickness is .020", that leaves a 1.96" ID. The thickness of the two motor tubes would be 1.90", which leaves only .060" or 1.5mm. It would be tight, but I think I could do it. I just don't know if it would be worth the trouble.
My idea is to be able to use two D12's or E9's, instead of having to mail order 24mm composite E's.
I would personally go the route of the 24/40 and 24/60 reloads. However, I understand the desire to stay with black powder. It lights easier, it's cheaper, and, heck, it smells better. (Don't tell me you don't know it smells better. :roll
If you have your heart dead set on the Interceptor E, then go for it. If you don't mind possibly building it stock, I would get the kit and check the fit of the two motor tubes. An easy way to do this would be to simply glue the two motor tubes together and see if they slide comfortably into the 2" tube. If they do, cut two centering ring halves (no, you will not be able to make them as one piece) and slide them on either side of the MMT to seal it up. I would also add positive retention at this point. A t-nut on the inside of the centering ring could provide retention for both motors; simply use a 2-56 or 4-40 screw and a washer to hold both motors in.
As an aside, if you decide to go the route of a two motor cluster (which I think would be AWESOME) I would use Q2G2's on every flight, just for added insurance. You don't want to put that much work into something just to crash it because a motor didn't light. Also, this would look awesome on two of the new Quest D8's.
Finally, if you want to do it, anything is worth the trouble. Just because other people say it can't be done doesn't mean that you shouldn't try.