Simultaneous Pressing Of Left And Right Mouse Buttons

I am trying to detect whether my IE6 users have pressed both right and
left mouse buttons simultaneously in my Javascript code, by using:

if(event.button==3) {alert("Both right and left mouse buttons
pressed");}

I am calling the above code on mousedown or mouseup events.

But it does not seem to work. :,(

The if condition is not satisfied even on pressing the left and right
buttons simultaneously.

Is there something that I am missing?