It actually is pretty easy if you just want to have one heal the other. I know you don't like cast sequences but it would really help in this case because it will help you if you decide you end up needing the one getting beat on to heal also.

You have two Shaman Healers; Shaman1 and Shaman2.

Shaman 1 Macros
(Below is for when Shaman1 is getting beat on, press twice if you feel that you need both Shaman to heal otherwise Shaman2 will heal)
lets say this is bound to Shift+1 for arguments sake
/castsequence reset=5 ,Healing Wave
/target Shaman1

(Below is for when Shaman2 is getting beat on, press once to heal Shaman2, if pressed twice then Shaman2 will also heal himself)
lets say this is bound to Shift+2 for arguments sake
/castsequence reset=5 Healing Wave,
/target Shaman2

Shaman2 Macros
(Below is for when Shaman1 is getting beat on, press once to heal Shaman1, if pressed twice then Shaman1 will also heal himself)
this would need to be bound to Shift+1 to coincide with Shaman1
/castsequence reset=5 Healing Wave,
/target Shaman1

(Below is for when Shaman2 is getting beat on, press twice if you feel that you need both Shaman to heal otherwise Shaman1 will heal)
this would need to be bound to Shift+2 to coincide with Shaman1
/castsequence reset=5 ,Healing Wave
/target Shaman2

Basically what the macros do is if you have both Shaman attacking something and one needs a heal, press Shift+1 once if Shaman1 is getting hit on and Shaman2 will start healing while not losing the enemy target. If you press Shift+1 twice then both Shaman will begin healing Shaman1 and then continue to beat on their target. The same goes for if Shaman2 is getting beat on, you will press Shift+2 depending on if he needs one or both Shaman healing him. The reset is in there because if you only need one Shaman to heal you will want the macro to reset so the next time you press it, the same Shaman is healing again without making the Shaman who did not heal the first time wasting mana/time healing. The reset can be tweaked if you deem it to be too long.

I am not at home so I am writing this from memory but this is how I set my macros up so they don't lose their target. Because you are targeting the Shaman to be healed after casting, it works the same a click. Have an enemy or nothing targeted at all and press your heal button and you will see your cursor icon light up waiting for you to click on a target. Once you click on your target, you don't actually target what you clicked on for anything besides that one cast.