Things to think about.
reset for castsequences ONLY resets after your last keypress, not an absolute time. So if you press a key anytime in that 30 secs, it resets the timer to 30 secs before your sequence resets. You might choose a small time like 12 or 10. The bubble debuff falls off in like 15, so give it some time for you to press renew once.

I HATE to actually target anything ever. I have to do it for meleeing, channelling, wanding and auto-shooting. But other than that, I don't like to target mobs. And I NEVER target players, ever. Each person has their own playstyle though. So I put my target in my macro options. Like:
/cast [@MainCharName, nocombat] Power Word: Shield
/cast [@MainCharName, combat] Renew
It bubbles out of combat, and it renews everytime after that if in combat. You can modify it to do more/less.
/castsequence [@focus-target, harm] reset=combat/target Shadow Word: Pain, Smite, Smite
/cast [@focus-target, help] Power Word: Shield
It resets after 3 casts or if you target a new mob. Assuming you have your main char focused on your alt.
It also (same macro) bubbles someone if your main targets them. So you can help in BGs, groups, etc.

You can use modifiers to do something like the same thing. There is probably 20 differents ways to get the same results.
/cast [mod:shift, @focus] Power Word: Shield
/cast [nomod, @focus] Renew

Another:
/castsequence [@focus-target-target] reset=12/combat Power Word: Shield, Renew, Renew
It does stuff to whoever your main's target is beating on. So it helps if your alt gets aggro or if your are in a BG or group. No thinking, no having 5 macros for each char, just one and it works off who has aggro. It bubbles. Then if you push it again, renew. Then renew again. By that time, it has been a few seconds (try to not spam buttons) and the bubble debuff probably has worn off. So it can either fall back to the start after 12 secs of not pushing that button (in case of end of fight or you don't need a heal often), or it rolls back to the start if you use up a few renews.

Hope that helps and good luck.