Application Gateway supports one internal IP and one external IP per application gateway, You can configure multiple listeners to check for incoming connection requests by using the port, protocol, host, and IP address
the Application Gateway v2 SKU supports static public IP addresses. The v1 SKU supports static internal IPs. An application gateway supports only one public IP address however you can deploy more than one Application Gateway resource to a single subnet.
I think what you are trying to accomplish is done by creating a multi-site listener within the application gateway settings. I am assuming the applications have a different URL or URI for access? If that is the case you can create a multi-site listener for each site and associate the correct HTTP settings and rules with each listener. This will allow you to use the same port but route to different back end host pools. I have set this up for a few clients and customers and it works like a charm. Let me know if you want further clarification. @Admin O365