i had it a little ago too.
Without multiclient i everytime freezed after some sec when i were ingame, but when i used only 1 core i didnt.
Now i have sometimes with Multiclient the situation, that when i want to switch back to the other client, it doesnt react. I mean i dont see anything, it remains black^^
But im still ingame and can accept partys with enter. But i get soon dc xD
Or when you dont have enough free memory and want to login your second char, sometimes your mem dissappears ^^, you will get ???? error on loading screen for 2. char. Then it helps to free your ram manually after you started the 2. client. But do that before login in the 2. char.
And a good thing is too, before login in 2. char or teleporting one of that chars, to higher the process priority before. After login or town you can set it back to normal.
But more RAM is never wrong^^
And if you get a Direct 3D error after ???? crash and say you wouldnt have enough video memory etc when you try to start a second client, theres a solution too. Try to start a other game (i use warrock) till the grafik engine loads, then go out of that game and you can start the 2. client without problems ^^