1) Because of my chanter my ranged companions do 100% more dmg. The lvl2 faster reload chant is amazing. 2) You can have more than one chanter in your group, and they can run different chants.
I only have one, I stopped bothering with the offensive/defensive chants, and I just have the ranged dmg buff one running permanently. My 4 ranged companions all stay in one group at the back and get the buff, while my 2 tanks are at the front soaking up damage. Works like a charm.