You'd just end up with a ton of overhealing, and not enough healing on the people who actually need it.

One way to multibox healers in raids is to have 10 separate buttons, 1 for each raid member, per character, so 20 buttons in total.
By "buttons" i mean keybinds. The easy way I do it is have a separate action bar page of 10 heals (buttons 1 to 0), manually type in each person's name (nightmare) and make shift+1-0 cause the second healer to heal a particular person.

I'm sure someone has thought of a much better solution, but simply healing the target of your target's target or whatever will rarely ever be useful.