Thanks!

Are you talking about 8-man content where there are two healers in the party?

In that case, it is like how jamesoakes said. If it is an older fight and people are mostly overgeared, one healer can solo heal the fight.

However, there are times when one healer might die and that secondary healer becomes the new primary healer.

There are fights that can be pretty punishing and if there were only one healer and that healer dies, it'd be extremely difficult to get through the fight.

It's why we also have two tanks. Apart from tank swap mechanics, a second tank is there to ensure that there will be someone who can tank the boss should the first tank be killed.