As others said before: DON'T
cause of many reasons.
But instead what about heavily using JavaScript to open a new window for each form you want just one by one and when the user once submitted the window will close and the focus will go back to your mainWindow.
so it'S alike :

windows open  focus has      event
============  =============  ===== 
index.html    index.html     user clicks "register now"
                             open new Window f1

index.html    newWindow f1   user fills out the form
newWindow f1                 and submits (newWindow closes)

index.html    index.html     open new Window f2

index.html    newWindow f2   user fills out the form
newWindow f2                 and submits (newWindow closes)
                                 
index.html    index.html
[download]

Well and here comes the interesting part, cause instead of index.html the browser could point to your script. Ok, it's a quick and dirty idea but I guess it's worth a try.

Have a nice day
All decision is left to your taste