Dolby themselves don't appear to endorse placing upfiring speakers anywhere other than at the front of a room. Yes, the likes of Denon and Marantz do offer users the option of locating a sinfle pair of upward firing speakers at the rear of a room relative to the location of your surrounds, but this isn't something Dolby would agree with:
Dolby Atmos enabled speaker positioning
We recommend installing four Dolby Atmos enabled speakers whenever possible. The use of four speakers will make the placement of overhead sounds more accurate, and you’ll get more precise, realistic sounds when an object, such as a helicopter, passes overhead. Two of the Dolby Atmos enabled speakers (whether they are integrated speakers or add-on modules) should be placed in the left front and right front speaker locations of your system. The other two should be positioned ideally in the rear surround speaker locations or alternatively in the surround sound speaker locations.
If you are using only two Dolby Atmos enabled speakers, place them at the left front and right front speaker locations.
Note: With the exception of the center and center surround speakers, all speakers in a Dolby Atmos playback system (listener level, overhead, and Dolby Atmos enabled) must be added in pairs.
Placement height
For optimal effect and to minimize direct radiating audio at listener level, place Dolby Atmos enabled speakers at or slightly above the height of your ears when seated. Avoid placing the Dolby Atmos enabled speakers higher than one-half the height of your wall.
Note: For some Dolby Atmos enabled in-wall speakers, if the placement is above one-half the height of your wall, you may need to adjust the vertical angle, if the manufacturer implements this feature. Please consult the manufacturer’s user manual for guidance.
Placement relative to listeners
To avoid an unwanted proximity effect, make sure the speakers are at least 3 feet
(0.9 meter) away from listening positions, ideally 5 feet (1.5 meters) or more. This distance may be less than 5 feet if the upward-firing driver(s) of the Dolby Atmos enabled speaker is placed well above the level of the closest listener’s head.
Positioning of add-on modules
If you’re using add-on modules, place them either on top of the front and surround (ideally, rear surround) speakers or within 3 feet (0.9 meter) of those speakers. Place on-wall or in-wall add-on modules in the same position. Dolby Atmos enabled speakers should be mounted so that the driver is facing toward the ceiling.
The most natable extract in this instance would be
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If you are using only two Dolby Atmos enabled speakers, place them at the left front and right front speaker locations.
Sorry, but no I don't know of anyone of hand who has located their signgle pair of upfirers relative to the surrounds as opposed to locating them at the front of the room. I'm sure that somone will have done this though? Whether it is as effective is another matter????