Should I use dual woofer? MTM? 2, 2.5, 3 ways speaker?

In 2 ways it’s not a good idea with two woofer as the two acoustic centers will be further so we will create lobs, see here the difference of vertical off axis response with one woofer then with two woofer in two ways :

On axis no change, but vertically off axis, there is cancellation, so you will have the listening sensation that the horn and the woofers play separately at close or medium distance listening position.

A MTM layout will bring a similar problem and will need to cut lower for avoid it.

Putting two woofers horizontally will also brings the same problem but worse, an horizontal accident.

The solution if we absolutely want bottom vertical or horizontal dual woofer placement is the 2.5 way :
With the first bottom woofer crossed with the horn and the other one crossed lower when we loose the +6db that give us the baffle (a baffle is a kind of 180° horn), on axis it will be :

Context: with a subwoofer, that is a very good idea, the interest of dual woofer or MTM is very limited, we use it if there is a SPL max limitation that have his origin in the use of a single woofer.

Conclusion

The regular 2 way + subwoofer is a good solution for close/mid-distance, like a combination of the bi-radial with a 15" vented.

3ways + sub will be useful more for long range with high SPL usage. But this approach bi-radial + 12" waveguided sealed + 18" vented works in medium range listening distance :

For MTM we will avoid 2 way and use 2.5ways filtering, with a look’s like the JBL K2 6500 :

But in 2.5way filtering the JBL K2 9500 is a 2 way speaker.