Why the need to do it with two sheets? Surely you just set the one sheet up to be the template that you want, then print to PDF once completed?
In any case, don't you have an accounts system like Xero/Quickbooks etc. to produce invoices?
I probably haven't explained it very well. The first sheet is for invoices that go out as paper copies and thus are manually printed and go out on the headed note paper, the second is for when the Client either loses it or queries it and asks for an electronic copy which thus needs the company letterhead and footer to be added as a Jpeg and then the whole shebang Pdf'ed.
As we only do c.10-15 invoices each month specialist software doesn't seem worth it.