The hotkey is defined in the RetroPie controller config. You can then customize the hotkeys in RetroArch.
Or I believe you can set up "shift key" behavior in the I-PAC config to do the same thing.
If you still want separate buttons that's fine, was just a personal preference to have my controls be minimal.
I haven't installed any leaf buttons/sticks myself, I guess you can check the other forums here or google around for more info.