I would think doing the buttons like this would prevent the 'over healing' issue that you're concerned about:

1 - Priest 1 PW: Shield
2 - Priest 1 POM/Renew
3 - Priest 1 Heal
4 - Priest 1 Flash Heal
5 - Priest 1 Greater Heal
6 - Priest 1 Prayer of Healing

7 - Priest 2 PW: Shield
8 - Priest 2 POM/Renew
9 - Priest 2 Heal
0 - Priest 2 Flash Heal
- - Priest 2 Greater Heal
= - Priest 2 Prayer of Healing

I certainly wouldn't have them targetting the same party member for heals unless there is only one mob in a pull and therefore only one person has aggro. That's why they would have different Focus Priority Lists. Also there is a 3rd healer who would be doing party heals. So in the LIKELY event that at some point during the raid a mage pulls aggro off of the off tank, for instance, the mage won't be totally hung out to dry.