Depending on the content I do one of two things:

1: Both tanks are taking different mobs. In this case each healer picks a tank (Good example of this is any example where an off-tank may tank adds, such as Coil turn 1)
2: Heavy party damage is expected. In this case, one healer focuses on the main tank, while the other focuses on the party (Good example is during Garuda, when you have three mobs spamming AoE in a small area)

Regardless, both healers should be aiding their own as they can. For example, a WHM focusing on the party should still keep Regen up on the tank, and a SCH healing the tank should still toss Sacred Soil over the DDs before a massive AoE, things like that.