Holy Paladins are the Tank healer, they can maintain both tanks alive by themselves with big heals, and still help with raid healing.

Rotation for Holy Paladin Wotlk

Healing Priorities

Keep beacon of light up on the off tank or Main Tank and spam holy light as needed > Keep [Judgements of the Pure] up for 15% haste >  Blessing of sacrifice when needed > Blessing of Salvation if a DPS has too much threat > Divine Shield if in danger > Blessing of Protection to save another player

Tips

If in need of mana and no real danger of doing so, melee the boss for extra mana with Seal of Wisdom.