Hi @Jérôme ,
Welcome to the Microsoft Q&A Platform. Thank you for reaching out & I hope you are doing well.
I understand that you would like to host two applications behind a single App gateway.
This is achievable by using Multisite Listeners.
You have to create two Multisite HTTPS Listeners each with the respective FQDN and certificate.
This document should help you to set up the Applications,
Refer : Create and configure an application gateway to host multiple web sites using the Azure portal
Thanks,
Kapil
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