Single Casement Window into Double Casement Window

[anthro2019]

Hi,

I downloaded a single casement window rfa file from pella.com (see attached), but my window needs to be double casement window.
I opened the rfa file and tried to make it into a double casement window by:
1.) Extending the wall longer (to fit a double casement window)
2.) Using a mirror command to mirror the existing window so that it would become a double window

But I got all these error messages.

What are the steps to convert this into a double casement window?

I'd appreciate your help!

Thank you!