2.2 Main- and supportive healer model
Healing generally happens in tandem. But not all content requires two healers to be constantly healing as this will cause overhealing
http://www.find-demo.com/ff14/handbo...ing_output.png
Each encounter has a certain healing output requirement. When this is not met, your party will slowly die till the damage simply overwhelms the healing output. When this requirement
is met, any output that exceeds the requirement would simply be overhealing. Overhealing isn't necessarily a bad thing, mind you. After all; overhealing doesn't kill anyone and in each encounter there will always be a bit of overhealing. For those strifing for greater efficiency, it's good practice to suppress the amount of overhealing to a minimum, allowing you or your healing partner to spend more time and resources into other activities mentioned as secondary objective.
With that in mind, you should be aware who's the main healer and who's the supportive healer of the pair. The main healer would take the greater bulk of healing for both tank- as well as group healing. The supportive healer will focus on other tasks while aiding when necessary.