Hi,
Yes - with multi-tenancy you can allow each customer to customize their own entities, menus, tool bars, access web services, etc. The items is question would be custom aspx pages and plug-ins. For plugins and custom aspx pages you will need
access to the full server and this is typically not allowed in multi tenant environments.
If you only have two clients that you would be offering hosted crm to have you thought of partnering with anyone. Some offer really cool stuff for free. :-)
Alex Fagundes -
www.PowerObjects.com