I can't believe your first PC would lag in Dalaran with only 2 copies running. I ran that exact config with 4 copies on 4GB RAM and as long as I gave it a few seconds to load after zoning to Dalaran, they wouldn't break follow.

I'd definitely upgrade the 2nd PC though.

[edit] Wait, are you running ISBoxer with full-size graphics settings on both? That would kill either one of those PCs, IMO.