question regarding macros with focus target for healing and /assist mainshammy for dps

hello all,
Short said

i have been ripping my hair off to find a way for me to cross dps/healing in raids for mc/onyxia usage nwo and future raids
considering names of tanks will chance from time to time i am thinking some sort of focus macro migth work for example.
maintoon 1 and maintoon 2 will have someting like this "just example of how i think i migth pull it off.

/focustarget "selecing maintank

/assist silvertachi "name of my main shammy
/cast ligthing bolt "for example.

slave 1,2,3
macro one
/focustarget "selecting offtank

macro two
/target focustarget?
/cast chain heal

macro tre
/assist silvertachi "name of main shammy
/cast ligthing bolt

macro's like this could make it easier to both multiheal if i can come as heales or cross dps with offhealing when required.
only problem is that i do not know if this or someting close to this macro config will keep the focus target as a focus target for me to cross heal/dps and still be flexsible when it comes to healing my main targets aka /focustarget
Have anyone tested out any macro's or addons that can solve my little hair ripping issue. have tried to do a google search for macro's and checked out the wiki section. but i fail to see any macro's or solution that could work as far as i see it.
sorry if i have explained it badly, but this sort targeting macro's is a bite confusing in my head so hard to type it down in a manner that is understandable

silver