How will multiple portals look on the end-user side? Will they have to login with different credentials?

Multiple portals will look like two different websites from the customers' point of view. 

Once a customer is signed up on a portal, he/she can use the same credentials to log into the other portal as well, depending on how the URLs are exposed. You would be able to determine the portal access by changing the user permissions in Admin -> Portals -> settings. 

 Please navigate to Admin > Workflows > Email Notifications > Requester Notifications > click on insert placeholder and include the placeholder for product-specific URL. This would allow customers to navigate to the support of the appropriate product.