Because Server1 is already syncing with share1, the Storage Sync Service and the Sync Group (which contains the Cloud Endpoint linking to share1) have already been created and configured. A Cloud Endpoint (the Azure file share) can only be a member of one Sync Group, so you cannot add a new Cloud Endpoint for the same share.

To add a new server (Server2) to this existing topology:

1. Install the Agent: You must first install the Azure File Sync agent on Server2 to enable it to communicate with Azure.
2. Register Server: You then register Server2 with the existing Storage Sync Service. This establishes a trust relationship between the on-premises server and the Azure resource.
3. Add Server Endpoint: Finally, you create a Server Endpoint in the existing Sync Group. This specifies the location on Server2 that will sync with the Azure file share (share1).

Microsoft Learn - Deploy Azure File Sync:

Section: Install the Azure File Sync agent

"The Azure File Sync agent is a downloadable package that enables Windows Server to be synced with an Azure file share."

Section: Register Windows Server with Storage Sync Service

"Registering your Windows Server with a Storage Sync Service establishes a trust relationship between your server (or cluster) and the Storage Sync Service."

Section: Add a server endpoint

"A server endpoint represents a specific location on a registered server, such as a folder on a server volume."

URL: https://learn.microsoft.com/en-us/azure/storage/file-sync/file-sync-deployment-guide