With two woofers in close proximity playing the same material, they essentially become a single source which appears to your brain to be located directly between the two. With an MTM, that means the woofers and mids appear to come from exactly the same place (the middle), not too unlike a Duplex.

With both woofs vertically below the horn, the apparent source of the LF material (again, between the woofs) is now at least 20-24" away from the location of the mid source. That results in you hearing both sources independantly, and they won't appear to blend into a single source until you're at least 25' away from them. That's why in our semantic discussion of vertical vs horizontal, horizontal might be preferable over vertical, even if the Q is less of a match, since it puts the apparent sources closer together.

When they were used in theaters and venues where the listeners were at a distance, the relatively large scale (distance from LF to MF) of Altec systems didn't pose a problem. Put them up close in a living room though, and you can easily hear it.

For the tweets (if you must use them) you could put them at the bottom of the top woofer cab, and symmetrically to the side of the woofs to keep everything close together.