Instant does support this feature and we do support multi-hop (out to 2 hops) on all of our dual-radio 802.11n IAPs (mesh for .11ac coming soon).
Mounting, as you described, with the AP edges pointing at each other is fine -- it will yield similar results to ceiling mounting as far as the mesh link is concerned.
Successfull Meshing, however, depends on the quality of the signal between the APs. Antenna mounting, external interference, etc must be mitigated for a successful deployment. If your case, with wall mounting, an AP model that uses external antennas, such as the IAP-104, IAP-114, or IAP-134 with AP-ANT-1Bs may yield a slightly better result as they have slightly higher gain than the internal antennas of the 105 (4.5dBi v. 5.8dBi). Again, however, without knowing if there is a rough RF environment, this could still be an issue.