Hello there,
The description you gave sounds like it will work
To recap what I interpretted from your post:
-P1 will have internet access.
-P2 will be the proxy IP for your clients.
Just be sure that you configure the routes correctly to have P2 speak to the internal network, and P1 speak to the internet if either of them will be restricted access to either the internet or the intranet.
-Vance