Hi Chop, I have put a little more thought into things and picked Mark Seaton's brain a little. I think I have come up with a plan that can work well for everyone. I'll detail that to you later on as soon as I finalize my thoughts a little.
Here are some images of the MTM design that used a pair of TD15M and the XT1464 horn with the BMS coax compression driver. I really do like the BMS coax. It is the best compression driver I have heard by far. Being a coax does present another passive crossover and getting that right is critical. The crossover that came with it just didn't work at all. We got 16ohm drivers instead of 8ohm, but the crossover was for the 8ohm so it wasn't even close.
Here is the image of the cabinet. This was very simple, made quickly and didn't factor in having nice grills or anything. They did a lot of shows though.
Elite's system was modular so they could bring in a lot of different things depending on the size and shape of the venue. It has some limitations but still worked well overall. In this image here the room was so wide they made 2 extra cabinets for side and center fills. The horns were turned to be only about 40 degrees wide to be more directional. This was for a 2 day metal festival at the Brown County Arena that had several thousand people there.
This was for an outdoor festival. Just the MTM's with the 2 TD15M's and the horns would cover an area of 1500 people outdoors very well. The single TD12M's with the XT1086 horns were used for center fill. The same system was used for full front of house for Saliva, Nonpoint, and other heavy rock bands at the Riverside Ballroom which I believe holds about the same number of people
Anyway, point being that these can get extremely loud and take a lot of abuse.
As far as the MTM alignment goes, It does have some benefit of keeping some of the sound off the floor and ceiling. The drivers will sum very well in the middle at the height of the waveguide. As you go up and down, the difference in time between the drivers will begin to give you some cancellation. We found that you have about 15-20 degrees height to work with on average. So +/- 7.5-10 degrees from the center. That is based on the center/center distance and the crossover point of 750hz used in these. Pushing the crossover lower helps to widen that a little. Even at +/- 10 degrees by a distance of 8ft from the speaker that is 32" of height to work with. At 16ft away that is 64". If the horns are centered at ear height with your front row, that should cover any range of seating, and a the back of the room easily cover sitting and standing height. The other benefit is that both drivers are the same distance from the waveguide then. So your time alignment would apply properly to both drivers.
In the real world though, whether you do MTM or TMM with the 2 woofers underneath can both work with a crossover about 500hz. If you were to go with a pair of TD15M's in about 10cf tuned around 55hz, you can get some serious output out of it. You can put in 1000W to the pair of drivers before running into excursion limits with about 133dB modeled at 70hz. This is what the Elite cabinets are tuned to. It actually gives a little boost from about 60-120hz that can be pulled down and F3 about 51Hz.
The interesting thing is that the TD18H's model nearly the same in that same enclosure. Efficiency is nearly the same with both. With similar tuning they can handle much more power without ever reaching excursion limits. This also leaves the option to tune significantly lower. You could even tune in the 30hz range and have a killer system to use without subs. The center to center distances are slightly farther you are really adding only about 4" between centers. The SEOS24 is creating most of the center to center distance, so that extra wouldn't be too critical. The TD18H is quite flat up to about 2KHz really as well so it is an option. The green curve is the TD15M, blue curve is the TD18H with same tuning and the white curve is tuned about 35hz.