MainAssist has a Fubar control widget, so I'll be referring to that.

1. Target one of your toons -- he must be in the same raid or same party though
2. Right click on the MainAssist widget in the Fubar
3. Go to Custom-->Target, and click
4. Right click again on the MainAssist widget in teh Fubar
5. Make sure there is a checkmark next to the "Show Tanks HP bar"

A draggable frame with the name of the toon you have been targetting with their percentage health should then appear.

Repeat as many times as you would like.