Custom Login page that passes control to IFD on CRM 2011?

I am using CRM 2011.

I have two active directories X and Y

CRM 2011 is tied to active directory X.

I have a one-way forest trust between X and Y such that Y users can use CRM 2011.

I wish to do a IFD.

I wish to build a custom login page that other applications that use active directories X and Y can authenticate against.

My question is ,after my custom page  authenticates against my X or Y users ,is there is a way to then pass control to CRM 2011 on IFD to take over?

Is there a website that has these instructions?