concentrating on only players with low HP is not possible.. It used to be under macros, but blizz removed that functionality back in 2.0 methinks.

I can explain briefly what I do if you like

ur main will likely take 60-70% of the damage under most circumstance, as the common "target the brain!" comes to mind. or in PvE u normally pull with said character.

Thus I have the following macros

1) 5xlesser healing wave on main (usually = 6k hp without crits)
2) 5x Chain Heal on main (the advantage of this is that under the circumstances where ur main is taking damage and your whole group is suffering from an AoE, your clones will also likely be healed)
3) Cycle chain heal - Each shaman targets another thus forming an equal healing circle (also if your shammies are varying in hp. the next jump on the chain heals will automatically jump to the next person with the least amount of hp - thus automatically healing the ones that need it more)
4) Lesser heal self - important for an emergency group heal where every second counts (it may also be a good idea to have a mod on this so that it works as tho each shammy is targeting another one - because the one that's taking damage is far more likely to be getting pushback and thus unable to heal itself)
5) target=Maintarget lesser healing wave/chain heal

lastly I have two macros designed for something along the lines of what you were saying

1) Chain heal as follows -- clones 1 and 3 target clone nr 3 ||| and clones 2 and 4 target nr 4 ---- this way clones 3 and 4 will receive 2 chain heals. while the main does a target=player one on himself for added protection of the brain.
2) Chain heal as follows -- clones 1 and 3 target clone nr 1 ||| and clones 2 and 4 target nr 2 ---- this way clones 1 and 2 will receive 2 chain heals. main does the same as before.

The reason I have it set out like that is that more often then not in PvE I have my clones in line formation, and one side of the group is likely to take localised damage and thus i separated left and right. however it won't really work well if say clone 1 and 4 are being damaged :S

It has a limited use