Hi Robert - (please forgive my delay in responding. I didn't get a notification that there was a new message)
In the hopes that this is not too late -
Hopefully, you're using an ensemble format so that the marginal identifiers will let the singer know which part singing which notes. When the duet drops out, just go back to a single part for the soprano.
I would have the duet part on a separate line(s) as the soprano 1 so that each measure can be Brailled in full and can easily be counted and sung.