In a properly designed system the mids should not overpower the tweeter regardless of configuration; no driver or group of drivers should overpower the others.
Orientation should be vertical for best sound, as with any speaker. Any array of drivers (even if it's just a woofer and tweeter) along a line will exhibit "lobing" (off-axis frequency response unevenness caused by interference patterns between the drivers) along that line. Keeping the alignment on a vertical line puts the offending anomalies above and below the speaker (the least offensive place for them).
Yes, that goes for center channel speakers, too. The only reason they're horizontal is to fit atop a TV. It's a poor tradeoff, IMO.
What are the stated advantages of an MTM or D'Appolito array, anyway? I should look that up...