I'm doing a holy and shadow priest together right now. I use the holy priest as my main, and have the Grid and Clique add-ons for healing the party. That lets you heal without switching targets, (right click the grid frame for lesser heal, shift-right click for greater heals, etc). I keep the DPS target as my healers target and the DPSer macros assist off the healer. It probably wouldn't work so well with Melee DPS, since that requires a lot more movement and control of the DPSer. they're only in the 30's now, so I don't have all the spells obviously but currently I just pass the following to the Shadow priest.

DPS keys [ Castsequence (Vamp Embrace, SW: Pain)
] Mind Blast
\ Mind Flay