It's entirely possible. And there's a couple ways to do it.

One way is to have both healers targeting the same toon and heal, making your slave just a carbon copy of your main. That would mean your main has to physically target the person they want to heal, then the slave would either assist your main and cast their heal or your slave would keep your main targeted and use /cast @targettarget heal. Basically the same setup as you would use for DPS.

Another is to use some sort of region and set it up so both toons get the heal commands.

http://www.dual-boxing.com/showthread.php?t=33614
http://www.dual-boxing.com/showthread.php?t=30511
http://www.youtube.com/watch?v=6qHo0FTuGV0
http://www.dual-boxing.com/showthread.php?t=28503