What ever speakers are used, if it's passive, the success of box will largely depend on the balance brought about by a finely tuned x/o.
A 2x12 mid will probably suffer from phase cancellation in the upper frequencies of the 12's, and there's no easy cure.
Tony, do you ever do 2.5 way crossovers? I mean if you've got a dual midrange, roll off the bottom one lower than the one closer to the HF so that only one driver is covering the region where it crosses over to the HF? Since a lot of drivers having a rising response you might get a flatter response?
Chris, I've never done that, but just to clarify, I was referring to the upper part of the 2 X12 regarding phase issues. It's the upper part of the 12's that go out of phase with each other when they are close together.. You may well have realised this, but I wasn't sure.
When I first looked at the drawing which was for about 10 seconds, I saw it as a straight baffle front loaded reflex job. It just goes to show what a plank I can be sometimes. So, I guess the division separating the speakers is the answer.
I have mentioned before, that there is a point between 2 speakers where coupling ceases to be obstructive and becomes productive, I always meant to attempt to draw a graph indicating where the crossover point might be. I think the difficult part is the fact that, although frequency doesn't alter, I believe the size of the speakers play a roll in the effect. This is because of the beaming effect, which happens when the length of the frequency being played reaches the piston diameter of the speaker, therefore, a 10" speaker will play much higher before beaming occurs compared to an 18"speaker where beaming would occur at a lower frequency
My point is, regarding the coupling effect, is that I believe it would happen at different frequencies according to what size speaker is used. Perhaps the answer is to calculate at what frequency a particular speaker will beam, calculate the length and make the dividing wedge slightly longer than the figure. Someone who knows more than me may have a better answer.