Spacing them closely like that causes no problem for them to get enough fresh air through their intakes.
Being on their sides, they get cool air from along the right front edge and top side.
The warm air is exhausted out the back and because of the angle they are placed at,
it forces the warm air to move down the wall to the left as it rises, so no heat can wrap around to the front.
The ceiling is 18' high along that wall and that room has over 1,000 sq/ft of open space, so I don't believe it will cause any problems.
I'll keep an eye on it tho, thanks for the warning.