It is 100% definitely better to get MU and SF up during TA, than do 2 AEs.

IR is used early because the AE used with Duality cannot crit, and when used after the first GCD, IR will drop just as you press AE, meaning the none of the duration of IR is wasted on weapon skills that cannot crit. That opener I posted is pretty much as optimal as you can get in terms of oGCD placement.