Two accounts wow at same time, one will lagging, need help

Hi, i'm new to dual-boxing. I have met a problem while i normally ran two wows on one computer.I use two accounts to log in to same realm, one of the accounts will must be lagging and disconnected soon. If i log in different realms, no lagging at all. I have tried to play on both same computer and two computers, problem still happens. My net is ADSL2, also a D-LINK router to share the internet. Does anyone have the same problem or know what's wrong?