Listing in both AppSource and Azure Marketplace
Great question! Cross-posting functionality is limited to SaaS applications (or, "web apps" as they appear in the AppSource storefront).
This is because a built-on Azure SaaS application could be relevant for both IT and Developer decision makers as well as business decision makers. Azure Marketplace is targeted to the former and AppSource to the latter.
To set your app for success, you should evaluate these things:
1. Is it a SaaS app? If yes,
2. Is it appropriate for both IT Pros and Business Decision Makers? If yes,
3. What are the correct categories, and correct keywords to help it appear in searches in both storefronts to the customers you want to find it? Make sure you input those in Cloud Partner Portal.
I recommend you click around in both storefronts to see how the categories filter and change the discovery experience.
It seems intuitive that you'd be able to broaden your reach by publishing to both, but it really hinges on capturing the right end customer. If your app is really IT Pro focused, it might not get much traffic in a space designed for Business folks.