Embedded forms and relationships

Is it possible to take data submitted on a form embedded in my site and compare it data from a different form in my application that is not embedded in the current page?

For example, I have one form in my application that takes data for creating a profile, which includes a username and password. I want to embed a different form in my website that will ask for username and password, then check the first form to make sure those records exits before going to the next url.

Possible? If so, how? THANKS!