how cloud I open GUI of my off-hand item when I hold a item in main-hand

when I hold items in both of my hand, I use procedure "open GUI for event/targe entity pass location xyz" to open a GUI, but every time it open the GUI for main-hand, even it doesn't link to a GUI .

So my question is , how cloud I open a specific GUI of an item not juse held by my main-hand when I get an item in my main hand.