Posting form data to another form

I'm not sure if this is easy to do or even possible, but I have a somewhat complex login page within a site, but I would like users to be able to access it (login) from the homepage with just their username & password.

Is there a way I can pass the data input by the user from a form on the homepage so it inputs it into the the input fields on the actual form page and logs them in there?

This isn't able to be done in the way that you are thinking. You can't post to two different forms at the same time without using curl or writing a function to do it.

Depending on how the db is setup is setup you could set it up to authenticate on the same table and then write a function to login to set session and cookie data..etc.