You may be running two clients in Full Screen mode. If that's the case this should help:
Go to your aRO directory and open "dinput.ini"
Look over the file and change these variables' values to these:
MouseFreedom = 1
AutoFreeCPU = 1 (*Note* It does nothing on Vista/Windows 7)
WindowLock = 1
Now you'll be able to run multiple clients in pseudo-full screen, and the alt-tab switch should be faster.