If you're only going to run 2 I wouldn't do Shaman. I run 3 shaman and it takes 2 volleys (CL + LB) and an Earth shock to nuke something down. With 2 shaman you may be taking more incoming damage while you kill something. You might want to look at 2 hunters or 2 warlocks, something with a pet that can tank a mob for you. I also agree that with just 2 usually a tank/healer combo works best for little downtime and has been a better experience for me.