Customer Portal - Zoho Sites

I have a dilemma. I've created a software based on automotive business and I've built a website with the customer portal with login.

I have different applications and I'd like to use the same login portal with other applications as well. I heard that this is not possibile, however I'm trying to find a solution to my problem... Does anybody have an idea on how to use one login for every application I have like a regular software?