Hi friend,
Here , we should consider two points,
1. Coverage and signal leakage through the wall.
2. Channel and signal power of each AP.
For good coverage and for avoiding signal leakage, it is recommended to mount the AP to the ceiling, away from the wall and horizontal to each other ( as Cappali said use L brackets).
When APs are close to each other it is recommended to enable ARM to handle the Transmitting power and channel.
Therefore I would recommend you to mount APs to the ceiling, horizontal each other and enable ARM.
Hope you got clarity on this,
Please feel free for any further query on this.