You're leaving out the most important thing for dual boxing with your priest..... holy nova. Even if you have to do things raid specced, treat yourself to one non raiding point in holy nova.

I really like a good pvp spec with holy nova and reflective shield (actually get some aggro).

Once you get the hang of it you can solo through SM cath easily, just learning to watch your mana is really important (holy nova rank 1 to gather them all, then 2 until they are almost dead, then finish them off with two blasts from the highest rank).

The most important lesson I learned about using my holy priest to run my alt through instances was to NEVER EVER use anything with a cast time
its tempting to try and flash, but in SM Lib/Cath that will be the death of you (some of those mobs can kick and lock you down for multiple seconds). just keep up your renew and watch your reflective shield timers. Pain supression is also handy for this as is Power Infusion.

the other really important thing to ahve is /cast [target=focus] Power Word: Shield and keep your alt on focus at all times. The threat you generate from shielding them will pull them back to you (since remember holy nova is all but threatless... PWS on yourself will give you the reflection threat as well, which makes it lovely). Now thats low level dual boxing.

For high level dual boxing with my warlock, I use twin "lazy" macros, both keybound to 2, the warlocks is

/assist priest
/petattack
/trinket1
/trinket2 (Activatable trinkets are worth a lot more if you never have to think about them)
/castsequence Corruption , Shadowbolt, Shadowbolt, ShadowBolt, Shadowbolt

priest is

/trinket
/castsequence SWP, PWS, Smite, SMite, SMite, SMite

Now some of you will notice that those two timers dont line up, but it doesn't matter, it wont interrupt a shadowbolt or a smite if you hit it midcast. the power word shield is to frontload 2k aggro onto the priest because its just preferable to me that she take the brunt since she's the main in actual gear. Actually I'm not certain of the amount of aggro, I should check it on omen some time.

as well as my shield focus macro I also have renew and prayer of mending ones, (my instant casts) from 2v2 arenas that occasionally come in handy.