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One of the things to beware of when using multiple drivers to produce the same signal is that you need to make sure that the highest frequency they produce has a wavelength in air of (roughly) no less than 4 x the distance between the drivers.
So, for example, if your midranges are 15 cm apart, then the highest frequency they should be used for is:
344 / (0.15 * 4) = 573 Hz
The 344 is the speed of sound in air (344 m/s) and the 0.15 is the distance between the centres of the drivers in metres. Note that it’s not strange to find yourself in a position where this rule directly conflicts with the drivers’ capabilities. For example, it’s not hard to get to a place where the drivers are so far apart that they shouldn’t be used at a frequency higher than something that’s LOWER than they should be used for… (Like being 1.8 m tall, standing at the bottom of a 2 m deep pool… this also causes a problem… eventually…) This is why the (front) midranges of the 50s and 90s (and the (front) tweeters of the 90s) are clustered to keep them as close together as possible. (However, in those cases, we’re also relying on the natural directivity of the individual drivers, which, like I said above, are measured in 3D space.)
Of course, this is a rule of thumb and assumes many things (like omnidirectional radiation of the drivers at that frequency, and the assumption that you DON’T want them to cancel anywhere…)
IF you want to do something else – say, to cancel at 90º off axis for example, then you should put the drivers 1/2 of a wavelength of the frequency you want to cancel (still assuming that you have omnidirectional radiation from the drivers…)
I mad some simple animations explaining this here:
https://www.youtube.com/watch?v=1KGeBEWsahY
-g