You can have 2 AD connect servers, one in production and other one as staging.
Only difference between production and staging servers are that in production server there will be import, synchronization and export steps (one for each connector) in one sync cycle. But in staging server there is no export that runs, only import and synchronization step runs as part of one sync cycle.
If for some reason production AD connect goes down then you can disable staging mode on staging server and get it to production immediately.
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