Are you using Win Finale v27?

If I understand you correctly, you have several Finale documents that are duet scores, i. e. scores with only two staves.

Like a document with Fl 1 & 2, another document with Clar 1 & 2, &c. - right?

And now, for the sake of easier (fewer) page turns for the two performers, you need to separate the two staves, as two individual parts - right?

I would use

Document menu > Manage Parts…

I have one document with a duet arranged for say two flutes but also have in the same document 2 clar, 2 saxes, 2 trpts, 2 trombone, I’ve copied the duet into the other staves. Now I’d like to seperate and print each duet with both parts printed on the same page. For each duet it would like 2 staff piano parts. I hope I’ve explained it better.

Manage parts --> New Part. From the right column, drag the 2 staves the middle column. Name it.

You could also define a group (Staff menu -> Groups and Brackets) with each pair of staves. This would allow a continuous barline and any type of bracket. But this should be done before generating parts for these parts to appear under Edit Parts and in the Print window.