As the Presonus has separate left and right inputs and thru outputs, I would suggest that it's expected to be connected to line outputs from the Focusrite and if you want to send those same line outputs onward to the, e.g., PA, then you plug the PA into the Presonus Thru ports.

As the Focusrite has four outputs, you could also give separate feeds to the Presonus and the PA.

The Focusrite headphone outputs have, of course, their own independent volume controls. That would allow a total of six headphones. There seems to be no way to control the mix for individual players.

Jazz relative beginner, starting at a much older age than was helpful.
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