filtered lookup: apply query filter

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CAML

SharePoint2010

Hello,
I have a task to make a small helpdesk app :)
I have a "SITE 1" where I made a custom list "APPS" with a list of applications and other custom list "MODULES" with a list of application modules from the first list.
On "SITE 2" I have another custom list "HELPDESK". In the "HELPDESK" list I have filtered lookup column called "Application" in which I choose an application from the "APPS" list from "SITE 1". I have another filtered lookup column called "Aplication Module" in which I choose the module from the "MODULES" list from "SITE 1".
The problem is that when I select aplication module in the "HELPDESK" list I get all modules of all applications. I would like that column "Aplication Module" is dependent on column "Application" and that just shows the modules of the selected applications.
I guess I have the wrong approach ...

Thanks in advance

Tomek