I personally think , since your intent is to reup this piece , that it would be best to replace the dual-pattern piece of fabric on its' back , with a single piece , following the existing pattern(s) from it's (original?) tacks .
This will , by it's nature , possibly add a bit more to the cost of the project (matching/sizing) since it's a large surface .
In the end , you'll have a piece that's covered as it was intended to be by its' designers/builders .
I've seen these furniture items displayed in large period homes in the Mid-West Mississippi River areas , where rooms tended towards being very spacious , unlike many modern homes ... have seen some used in conjunction with a flat-topped & upolstered 'chest' , which could be placed behind the settee - for additional seating & backrest (with 'flat/straight-backed' settees) .
That's just one opinion ... let's see what others say !