How to do failover between AG setup for FCI-FCI servers
When you said it needs to be set as &quot;async&quot; as per limitation, are you talking about SQL limitation or limitation on your side because of, for example, network latency ?
I'm pretty sure you can build you AG in Sync mode if you want so, I will presume it's limitation on your side.
To answer your questions:
If the primary node for SQL1\inst1 goes down, the cluster will failover to the other node. (just like regular FCI) - Same thing for you second instance in DC2
As you AG will be in Async, the best approch to plan a failover from your DC1 to your DC2 without any data loss would be to
1- Have a maintenance windows where there will not be too much activity
2- Switch your AG to Sync mode and let it sync
3- Once sync (you can check dmv to make sure they are sync), trigger a failover within SQL (Using the Gui in SSMS or TSQL)
4- Once the failover is completed, set the AG back to Async
Note that in Async, you cannot enable Automatic failover so if ever your FCI from DC1 fail (problem with the shared storage for example) while being primary, it won't failover to your DC2 and manually failover to it may cause data loss.