Thank you for posting your query on Microsoft Q&A, from above description I could understand that you are looking for advisory on supported scenario for multiple Entra ID connect in single domain environment.
Please do correct me if this is not the case by responding in the comments section.
As per Single forest, multiple sync servers to one Microsoft Entra tenant, having multiple Microsoft Entra Connect Sync servers connected to the same Microsoft Entra tenant is not supported, except for a staging server. It's unsupported even if these servers are configured to synchronize with a mutually exclusive set of objects. You might have considered this topology if you can't reach all domains in the forest from a single server, or if you want to distribute load across several servers. (No errors occur when a new Azure AD Sync Server is configured for a new Microsoft Entra forest and a new verified child domain.)
Microsoft Entra Connect supports installing a second server in staging mode. A server in this mode reads data from all connected directories but does not write anything to connected directories. It uses the normal synchronization cycle and therefore has an updated copy of the identity data.
In a disaster where the primary server fails, you can fail over to the staging server. You do this in the Microsoft Entra Connect wizard. This second server can be located in a different datacenter because no infrastructure is shared with the primary server. You must manually copy any configuration change made on the primary server to the second server.
Thanks,
Akshay Kaushik
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