Just to let you guys know on this.
For group builds, Mantra of Healing procs Guiding Light.
So, running two Mantra in the supporting spec is completely viable.
I run this for vota group:
http://us.battle.net/d3/en/calculator/monk#adSfQk!fXZ!YbcZca1. Pop MoH when groups are around you. The radius of mantra is pretty big.
2. Pop MoC right after to get the overawe again
Repeat this whenever.
People usually use Breathe of Heaven or CS Soothing for GL buff, but BoH has horrible radius and cooldown and Soothing has small CS radius.
Enjoy.