I run a 70 prot pally and 70 holy priest combo (and am currently leveling a druid and mage for kicks). With proper macro setup (I agree 100% about it being very important to get the macros just right), I can dual tank/heal for any non-heroic instances with my guildies (who are very casual players and are almost all DPS classes).

I've seen folks with multiple identical toons, but that was never for me... glad to hear I'm not the only one