I started in the same position as you. I had 2 computers 2 keyboards 2 mice. I basically only used my resto shaman for his earth shield buff, didn't even bother grouping him with my warrior.

Since I found this site I setup a trial account and now have 3 accounts running on 1 computer. Using keyclone I know have 3 shaman running around melting face. It is a lot easier than what I was doing before, but it also has its frustrations. I'm not very high level yet, so I'm constantly running around picking up quests and quest items across 3 computers.

As for he old world instances, I have a MS warrior and a resto shaman. I have duo'd all the old 5 man content without much problem. Ive been trying to make a 45min baron run for kicks. I'm about 5 mins away from beating it.