You're over thinking this.

I use two disc priests in my level 90 team and I just use as one of my multiple step macro for disc priests "/castsequence [@focus] power word:shield, prayer of mending"

Then whichever of my priests has the highest spellpower, I have her make my tank their focus and leave the other one without a focus. One shield every 15 seconds isn't such a huge mana drain that you have to worry about spreading it out among your toons. Best to just get the shield from the strongest toon every time its up.