To expand on Boxi ng instances:

I ran a few BC instances as pally tank / disc priest. The best way I found to effectively tank and heal was to setup my pally tanks action buttons on the 1 through = , and my holy priests action buttons on the Numpad 0 through Numpad 6. (you can do more healing spells or damaging spells on Num7-Num9, /,*,- if you need them).

I used mod macros (an example below) for the disc priest so that Num0 would GHeal my priest, Alt+Num0 would flash heal, and Ctrl+Num0 would Shield. Num1-Num4 were setup to do the exact same thing with Part1-Party4. Num5 was CoH and Num6 was binding heal. (Note: You can't use shift modifier with your Numpad)
Keeping the party number matched with the numpad number made it easier to remember which target would be receiving the heals.

Num0 macro:
/cast [nomod, target=priestsname] Greater Heal
/cast [mod=alt, target=priestsname] Flash Heal
/cast [mod=ctrl, target=priestsname] Power Word: Shield

Num1 macro:
/cast [nomod, target=party1] Greater Heal
/cast [mod=alt, target=party1] Flash Heal
/cast [mod=ctrl, target=party1] Power Word: Shield

A couple other ways to do it:
1) Simplify your tanks actions by using the one button tanking methodwhere you could actually bind it to your mouse, and use your regular keypad for healing.
2) Have all of your tanking keybinds to 1 through = and all of your healing keybinds to be modifiers of those (Shift+1, Alt+1, Ctrl+1).
3) For simple macro solution you could make /cast [target=party1target] Healing Spell, you would just have to target who needed heals with your DK each time. (pita imo)