And I've seen cars with eight 6x9's in the rear shelf.
Just because it's been done, doesn't mean it was done correctly or intelligently.
As audiolife mentioned....I'd look at the entire setup. Maybe the sub and horns need toned down some to bring out the midbass. Or, maybe you need to set them up differently (sealed enclosure, etc).
Or, maybe you need to look into switching mids completely to something that will fit your tastes better.
But...all in all, I'd rather have one larger driver than two smaller drivers. More pointsources typically aren't a good thing. And, since they'd be playing the same frequency range, it will be harder to align them properly to maintain a good soundstage and imaging. All of this is much easier to accomplish with a single, larger driver.