You could have the priest remember who it heals. It wouldn't help if many people are taking damage, but if you need to cast several heals on the same person. I suppose you are using a standard assist focus setup?

If you use the priest mostly for healing, you can use "...[target=focustarget]..." for all offensive spells, leaving target for heals. This way you only have to target a friendly with the rogue when you want to switch targets.

  • first /target rogue's target if friendly
  • cast heal on target if it exists
  • If no target exists, cast on self (or on rogue)

Code:
/target [target=focustarget,help,nodead]
/cast [help,nodead][target=player] Überheal