Seeking advice on a single button for dps/hold shiftbutton+same key = all cast heal on that target.

[Maxion]

/cast [@target, noharm, mod:shift] flash heal
/cast [@target, harm][@targettarget, harm] smite

The second line could contain [@targettarget, harm, nomod] in it but it will never pass the first line as long as you have shift held down. I've not tried combining @target and @targettarget in the same macroblock so I'm not positive the smite line will work.

This should work, though is it possible to make one a bit like what Owltoid tried work too.
Something like:
/cast [@target,harm][@targettarget,harm] Lightning Bolt ; [@targettarget,help][@target,help] Lesser Healing Wave

Or something like that.
The only problem I can see is that the master may cast the heal on the targets target instead of the target, since you have to put the targettarget version first so the slaves don't just cast it on the master since they are targeting the master for their targeting instead of using focus, which would fix this issue.
Unless of course, you never change master, in which case you'd just make the slaves have a different version of the macro than the master.
But if your master isn't a shaman and will not have the macro themselves and the slaves will always be targeting the master, then all you need for the slaves is something like:
/cast [@targettarget,harm] Lightning Bolt ; [@targettarget,help] Lesser Healing Wave