When trying to focus a friendly target through a macro (/focus "Name") the target must be in your party. You could still make a non party member focus using the mouse but /focus doesn't seem to work anymore.

I had the same issues until I read somewhere (can't remember where exactly) that it became a semi protected function on prior patch to prevent botting.

Few Possible Solutions:
(brainstorming as I type)

Solution 1:
/target "Name"
/focus [target=target]

Then your heals:
/cast [target=focus] Healing Wave
etc.

You will need two different focus macros though, making it a toggle isn't possible as far as I know. Not that great since you will need to use two keystrokes one to set focus, the other to heal.

Solution 2 (easiest and most reliable):
Since your dualboxing, it might be easier to setup two different macros for each of your main healing spells (ouch upto 6 macro slots but it saves headaches).

Macro1:
/cast [target="WarriorNameHere"] Healing Wave

Macro2:
/cast [target="ShamanNameHere"] Healing Wave

Simplistic but it saves you from having to keep switching focus. And when crucial heals are involved, you don't want to rely on a toggle. This also allows you to keep your current target and focus so you can use dps macros if needed such as:

/cast [target=focus] Lightning Bolt

Solution 3 (works with the right skill but still messy):
You could heal targetoftarget. Basically when your shaman or warrior needs a heal you would select (with your warrior) the mob that is targeting him and then cast the following macro. Though I'm not sure how lag would play into this.

/cast [target=targettarget, exsists, help] Healing Wave