I have seen quite a few blog posts from late 2017 about running mixed windows and linux nodes on k8s in Azure on ACS.
https://anthonychu.ca/post/hybrid-kubernetes-linux-windows-cluster-easy-steps/
https://github.com/dunnock/kubernetes-hybrid-in-azure
https://chrisdennig.me/2017/10/25/deploy-a-hybrid-kubernetes-cluster-on-azure-container-service/
are several. All of them say that mixed container clusters are not available on AKS yet. It has been 3 months since the majority of those posts. Are they available on AKS now?
It is not available yet, please provide your feedback here: https://github.com/Azure/AKS/issues
You may wish to keep a watch on:
Azure Updates: https://azure.microsoft.com/en-gb/updates/
Azure Roadmap: https://azure.microsoft.com/en-us/roadmap/