Swoon, it sounds like you're using keyclone...I might be wrong. Since you're setting focus with the priest, why not /assist focus (1 macro button) so you can cast un-macro'd spells since you'll then be targeting what your lock has targeted? Acquire target with the lock, hit the /assist focus for the priest, start blasting. It's how I'm doing it with my 5x shammies. Of course, whatever buttom maps you have for your lock will need to be something corresponding on the priest with keyclone. If the lock shadowbolts, your priest could smite, etc.

Here's the macro:

/target focus
/assist
/follow
/stopcasting