Easy, as a healer, as soon as I cast a cure/ regen on the tank, it displays a list of the enemies the tank had an effect on (provocation, flash, whatsoever), so it becomes immediatly clear as crystal which enemy has its life decreasing faster than the other.
For any dps, it will display the list too, as soon as the player casts an aoe.

Anyway, just relying on enmity as a dps, is really blind. If every dps focus on a different enemy, it will always make the fights harder on the healer. Say, each dps can put an enemy down in a minute by solo-ing it. Tanks will get hit by the 2 mobs during a minute. If you focus the same enemy as the other dps, you can safely assume it will take 30 seconds to kill it. And another 30 to kill the second enemy, which, in other words, means that the tank (and possibly even the dps), get hit by half during the second part of the fight.
For most healers, it won't change a thing. For new players or slightly undergeared healers, it certainly does matter.