var leftButtonDown = false;
var rightButtonDown = false;

$(document).mousedown(function() {
    if(e.which == 1) {
        leftButtonDown = true;
    } else if (e.which == 3) {
        rightButtonDown = true;
    }
});

$(document).mouseup(function() {
    if(e.which == 1) {
        leftButtonDown = false;
    } else if (e.which == 3) {
        rightButtonDown = false;
    }
});

$(document).click(function() {
    if(leftButtonDown && rightButtonDown) {
        // left and right buttons are clicked at the same time
    }
});