Multiple Portals, does the user require multiple logins for two portals

Hello, 

If two portals have been installed, for example Customer Service and Project Portals, is the client required to have two separate login identities for both portals, or does one login cover both of them? Also if the client switches between the portals do they have to login again?  I have not been able to find documentation on this.

Thank you, 

Mahone