For two drivers I double the Vas, Halve the Re & Lvc, and double the Sd. I don't know if this is the correct way in MJK's worksheet, but that's the way I do it. The final product comes very close to the simulation.
Maybe Mr. King will jump in and give us his advice.
I also try three different positions for Driver Position Ratio to get an approximate idea how two drivers affect the response curve.
Modeling two drivers, located closely together, along a TL or TQWT is an approximation that is probably not too bad. Modeling a line array using my MathCad worksheets is definitely not very accurate. Different drivers are going to see different pressures and velocities on their back sides depending on where thay are located along the line. I would not recommend this method of modeling, a more exact worksheet that accounts for multiple drivers in different positions would be required.
I'm somewhat as crazy as you. I recently tried 2 - 5 1/4" drivers in a 112" TL. The box was 20 1/2" high 11" wide and 16 1/2" deep. It had 5 baffles inside to give it a tapered 112" length and a 3" round port 3/4" long for the terminus.
Dang if it didn't have some good bass that went down into the 40's.
The midrange doesn't sound as great. It might be that I have it crossed too high - 3500 hz.
I'm going to try a tweeter that'll go as low as 1500 hz. but that won't be til July.