Website A - Our website
Website B - External website (Gmail in this case)
When I go to Website A, enter login and password (As Gmail), I will see the items in my inbox of Gmail but with different GUI.
Timeline 4 month to 10 month.
Let me give you additional information....gmail/stock market is just an example..
1) website A -- stock market site(rates up and down every second of some company)
2) website B -- need to display as current stock price as possible.
3) We should be able to purchase share from site B. But eventually the purchase is done at site A as site B is not authorized to provide selling service.