Have you seen this:
It is a recent BIOS for the FW Desktop.
It mentions in the release notes:
6) Enabled 3A PD profiles on the back ports.
That might solve your USB4 enclosure problems.
Edit: FW removed the “enabled 3A PD” from the release notes, so it probably does not help now.
That being said, the FW13 and FW16, although they have 2 USB4 ports, they cannot actually provide much power to the ports. 15W to only one port at a time.
So, that might be why one USB4 device works and the second does not. Maybe not enough power is available for two at the same time.
The FW Desktop documentation does not say what power it can supply to its USB4 ports, so I don’t know for sure either way.
But, BIOS 3.03 might help you.