>Hi,
>below is the project structure. MainProject is the web app root.
>
>

>\Projects
>     \MainProject
>         \Forms
>             \MyForm.aspx
>     \SubProject
>         \Forms
>             \MyFormB.aspx
>

>
>Please advice. Thank you

As you found, I don't think it is possible to reference paths above the web root - which is what you would need to do to navigate from MainProject to SubProject. No suggestions other than redesigning the site layout I'm afraid,
Regards,
Viv