If you create two private (or public) ideas portals and both have product A and product B, any ideas created in either portal are visible in the other.
In this scenario, I want one portal for employees and one for customers. I do not want customers to see the ideas from my employees. I have no way to prevent this unless I make the portal public and create all customers as public portal users and my employees as private portal users. This is not desirable as I want a private portal for my customers.