The functionality you're looking for is the target= option. Like all options, just put it in brackets after the /cast and before the spell name. It changes the target for that spell and that spell only without actually affecting what the character is looking at. For instance,

/cast [target=focus] Flash Heal

Heals whatever your focus'd on without changing anything. Moreover, there's a wide range of things you can put inside.

/cast [target=targettarget] Renew

Drops a renew on whatever your target's target is.

If you're looking for more complicated examples, check the sticked post here:
Useful multi-boxing macros
And also this one:
Setting a New Leader Upon Death

Both have some examples of people using target= for exactly what you're talking about.