The two systems are independent. "Can Duplicate" refers only to the graphical hand pipeline through the Hand Model Manager.
Providers are the true source of hands, not the Hand Model Manager (formerly known as the HandPool), which simply takes hand data from the Provider and sends it to graphical representations (and does pooling, etc.) like Capsule Hands.
The Interaction Engine supports as many Interaction Hands as you can provide data for, but the workflow is set up to most easily support the first-left-hand and the first-right-hand in any given Frame from the Provider. If you inspect the Interaction Hand (Left) and Interaction Hand (Right) objects beneath the Interaction Manager, you can see they are set up to retrieve hands from the frame by their data mode setting: Player Left, and Player Right.
You can instead choose a data mode that is "Custom", which means you have to add a script to tell the Interaction Hand (in Awake() or OnEnable()) how to retrieve data from each frame. Player Left is simply "the first left hand in the frame, or null if there is none", so you could reproduce this, and add a second Interaction Hand that iterates through Frame hands and retrieves the "second left hand in the Frame, or null if there is none", and the system will act accordingly. There is documentation inside the InteractionHand.cs file itself for these custom-data-mode requirements.
With some apologies for supporting more than a single left and right hand in a frame at once; this was a workflow decision we made to support the VR use-case easily, which tends to be the most common.