Does only the primary charm's bonuses get added, like life per attack or max life? It seems like my second charm isn't taking effect.

accessory charm doesn't give you any listed bonuses
the only time it does anything is if you use it to attack or heal
when doing that it works at 75% efficiency, in catas it's mana cost is 75% too
EE does work on it to increase damage/heal
and then 75% overall after the EE

he summed it up. put your good stat charms in primary, the 2nd slot is usually for a heal or dmg charm with high EE.

it's pretty common to see Healers in Catas with the heal charm in the Acc. Slot, and the other 3 pieces of gear have Heal Mastery as the main mod. This does far more than INT ever could in regards to healing.