Lightroom-Classic is 'Old School' when it comes to syncing- something it was never designed to do, and will possibly always have limitations.
Correct, you can only 'Sync' one Catalog to the Adobe Cloud Ecosystem. If you try to sync another catalog, you can, but you get the warning message, and it causes complications when the active LrC Catalog attempts to (automatically) download (to the computer with the synced catalog) ALL full-size files that you have imported to the Lightroom Cloud.
LrC catalogs on two computers basically can not be 'synced', -the catalogs are independent of each other, although there are methods to use one catalog on both machines.
MULTIPLE COMPUTERS- How to use (Lightroom Queen)
COMPARE LIGHTROOM & LIGHTROOM-CLASSIC FEATURES
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