How do you wield the mouse when dual wielding? Do you mash both mouse buttons? Do you alternate as fast as your fingers will work? Do you alternate methodically or slowly? Do you press both buttons at the same time slowly?
You can do both, alternate for single attacks, fast click both buttons for a quick dual attack and hold both buttons for a power dual attack, or hold a single button for a single power attack, you can use any method.
Don't alternate between left and right click as it's slower and ultimately pointless. If you want a normal combo just keep clicking one button its faster than alternating. Hitting both quickly hits with both weapons for no stamina cost, don't know if this is more or less DPS than just spamming a single button though.
Really the problem I have with dual wield is that end game you just stand and do the both button standing power attack over and over and over and over.... since it does significantly more damage than any of the other power attacks. I'd like dual wield a lot more if there were forward, backwards and left/right dual wield power attacks too.