/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.