Hi welcome to the forum.
What speakers do you currently use in you 5.1 setup? What's your budget for the 2 atmos speakers? Or do you have some you plan to re-use?
Since you don't have the option of in ceiling speakers. These are the 2 configurations you can do to implement a 5.1.2 (Dolby Atmos) setup. (When using only 2 atmos speakers, they are always placed at the front.
Option 1:
Front Height
View attachment 1492475
Which as you suggest is placing 2 speakers high up near the ceiling above your front left & right speakers
Option 2:
Dolby Atmos Enabled Add-on Modules
View attachment 1492477
Where you place "upfiring" Dolby Atmos Speakers on top of your front left & right speakers.
For option 1 - you can pretty much use any wall mountable speaker.
For option 2 - Dolby recommend the following:
For optimal performance, the ceiling should be flat (not angled or vaulted), with a height
between 7.5 and 14 feet (2.3 to 4.3 meters), and made of an acoustically reflective material
(drywall, plaster, hardwood, or another rigid, non---sound-absorbing material). The ideal
ceiling height is between 7.5 and 12 feet (2.3 to 3.66 meters).
To improve sound quality and system performance, we recommend using sound-absorbing
and sound-diffusing treatment to reduce unwanted reflections from the walls and floor.
When using Dolby Atmos enabled speakers, first audio reflection from the ceiling is desired;
audio reflections from elsewhere within the room should be avoided.
Note: If your room has acoustic ceiling tiles, replace the tiles where the reflection from the
Dolby Atmos enabled speakers will occur with foam-core board or other reflective material.
Here are some good examples of Atmos Enabled Speakers:
Take your home cinema sound to new heights, with the Elac Debut A4. 2 Dolby Atmos speakers.
www.richersounds.com
Enjoy the sound of cinema in your own front room, with Monitor Audio’s first Dolby Atmos approved speakers – the Bronze AMS. Dolby Atmos approvedProducing a multi-dimensional and totally immersive sound, Dolby Atmos has been hailed as the greatest advance in surround sound for 20 years.
www.richersounds.com
There are lots of Atmos Enabled Modules.
Of the 2 options I would go option 1, as I find you get a better effect with front height speakers. In my setup I use some Dali Alteco C1's mounted near the ceiling:
The DALI ALTECO-1 adds Atmos, DTS-X and Auro-3D height information to a home cinema setup. It's also a discreet on-wall stereo speaker. Find out more.
www.dali-speakers.com
Shop for Dali Alteco C1 Black Surround Speaker Pair with Richer Sounds for a 6 Year Guarantee, expert advice and Lowest Prices Guaranteed
www.richersounds.com
Not the cheapest, but they sound great.
Here's what they look like mounted as heights on the wall:
View attachment 1492487
They can also be placed on top of your current front speakers like this:
View attachment 1492489
Many members also use Cambridge Audio Minx Min12's mounted high up on the wall.
Hope this helps. Feel free to ask any further questions!