Hello, Sorry to raise a 3 month old issue, however I'm also having the same issue as OP reported.
I have never used curtains, start 10 or start 11 before, and after a fresh first time install, I'm also noticing the same issue using a dual-monitor setup. I have 2 BENQ 2K monitors (2560x1440 each). Both are hooked up via Display Port connections, NOT HDMI.
Here is my windows 10 information:
Windows 10 Pro. Version 21H2 (OS Build 19044. 1706)
NVIDIA GeForce GTX 1070
This is what my display config in windows looks like (extended desktop):

Only the primary monitor gets the centered taskbar and not the secondary one, even though its extended. As far as I know, I don't have any other software installed that should be conflicting with Start 11.
I hope that helps in debugging this issue, I'm on a trial and was really hoping I could get this feature working on both taskbars.