Hello @Kasper S
Yes, you can create a new tenant for the external organization, create identities for your users in AD, create a new subscription with per-user access pricing, and deploy Azure Virtual Desktop in the new subscription.
You do not need to use AD DS in your situation, as you only have one external organization, and you are using per-user access pricing.
Please note that per-user access pricing only supports Windows 10 Enterprise multi-session and Windows 11 Enterprise multi-session. It does not support Windows Server session hosts.
Additionally, the per-user access license only grants access rights to Azure Virtual Desktop and does not include Microsoft Office, Microsoft 365 Defender, or Universal Print. You will need to separately license other products and services to grant your users access to them in your Azure Virtual Desktop environment.
Document source:
https://learn.microsoft.com/en-us/azure/virtual-desktop/remote-app-streaming/licensing
https://learn.microsoft.com/en-us/azure/virtual-desktop/remote-app-streaming/architecture-recs
https://learn.microsoft.com/en-us/azure/virtual-desktop/remote-app-streaming/overview
https://learn.microsoft.com/en-us/azure/virtual-desktop/remote-app-streaming/identities
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