From a Single Sign-On perspective, yes, you can use the same web agent and front end proxy server for both applications.
However, the question really should be directed at the two application admin teams. If you are using one host name in Apache for both applications, you need to make sure that there are no collisions for the proxy pass rules. I.e. it is always clear from the context root of the URL which application Apache should forward requests to.
If this is not clear, you could use two virtual hosts in Apache, and each virtual host proxies to a different application. If you need SSL, you may need a separate IP address for each virtual host.
You can get away with one IP address if your Apache server, and all your clients, support the "server name indication" extension in TLS. I believe it's supported in Apache 2.2.12 and later. Note, WinXP clients do not support it.