Hi Geo Thomas,
Refer to the C9500 datasheet, Stackwise Virtual is supported in a 2-node installation:
"
Cisco StackWise Virtual: StackWise Virtual is an advanced stacking technology that supports both access and distribution deployments through multiple topologies (such as two nodes or a ring). It provides higher scale for system virtualization at the network layer. The Cisco Catalyst 9500 Series supports StackWise Virtual with a 2-node topology on select models at FCS. Refer to the Release Notes for limitations.
"
