I would go for Gaffy's idea:
Code:
/cast [target=talorab, nomod] [target=taloraa, mod:shift] [target=dagbit, mod:ctrl] [] Power Word: Shield
This will cast Shield on talorab when you press the button , if you hold Shift while pressing the button you will cast Shield on taloraa, if you however hold CTRL while pressing the button, you will cast Shield on dagbit. I added a [] in case you want to cast it on your target.