I click the left once to switch to sights or not. But why not change it to what you want?
See picture at http://i175.photobucket.com/albums/w146/rproffitt2000/b2aim_zps36ad5b60.png
So I am attempting to play borderlands 2 on my mac. However, to look down the sights, I have to hold the right mouse button. To fire my weapon, I then have to press the left mouse button while still holding the right one. This ultimately fails as my mac will not allow both to work simultaneously. When right click is held, if I left click, this overrides the previous right click. Any suggestions?