SQL Availability group IO prioritisation How does SQL Server handle IO in terms of availability group secondaries?
To give an example scenario:
A 2 node cluster with 2 Availability groups
Each AG is primary on a different node.
Therefore each node is a primary for one AG but a secondary for the other.
Will IO for the primary workload be prioritised over the replicated data, assuming all disks are shared?
Also would this differ if the secondary was in sync or async mode?
Will IO for the primary workload be prioritised over the replicated data, assuming all disks are shared?

No, I have not heard any such thing that SQL Server will prioritize certain IO over other. You have an option to set High Priority for SQL Server by using sp_configure but it is not recommended and mostly counter productive.