Let's say you have Steadfast (one handed sword) as main weapon. Then you find Little Saviour (small shield) and want to use it as your main shield with Steadfast. All good.
Now you also find Aila Braccia (small shield) which is extremely good when you meet foes who shoot directly at you with missiles and spells - way better than Little Savior, but worse in melee. Now you want to switch between set 1: Steadfast + Little Savior (melee cmbat) and set 2: Steadfast + Aila Braccia (if you meet things like Lagufaeth with blowguns and stuff).
But you can't in PoE! You would have to choose a different main weapon. Or you would have to clone Steadfast - which isn't possible without the console and cheating (=deactivating achievements).
People wanted to define different weapon sets where you "mirror" the weapons or the shields. You will not have them twice, you just "virtually" place them into different weapon sets so when you switch sets one of them stays and only the other shield/weapon gets switched. Hope that makes sense for you...
Edited by Boeroer, 08 February 2017 - 02:05 AM.