Azure App Service Environments Internal and External access

There are multiple angles you can choose and ultimately your appetite for management, operational flexibility, and cost will be core drivers to the architecture. You will most likely find that two separate application architectures (with isolated compute) will be what most companies do as they would rather take on the extra work and cost to avoid mixing the two environments together and misconfiguring anything that could lead to a potential breach (especially for your intranet site). 

Write down your core requirements for each site and the constraints for each before making a decision. Leverage some of the guides from Microsoft on https://learn.microsoft.com/en-us/azure/architecture/web-apps/web-apps-get-startedand picking the right architecture to meet those requirements.