Graphical Lag with multiple Clients

Hi,

I've created a second account to level another character (not really multiboxing, just want to trade myself mage water, etc), but I get graphical lag when I have two TBC clients open at the same time. It basically stutters and drops a few frames every second or so. I tried turning all the video settings to low/off but that did not change anything. Wondering if anyone else has had this problem or knows how I might address it?

Also, I don't think it's my PC struggling. I can easily run Overwatch or retail WoW on max settings while playing TBC in the background with no graphical lag/issues. It's only with two TBC clients.

Thanks for any suggestions.