In Engrave mode:
Click on the Violin ossia staff, Ctrl/Cmd-click on the Cello staff, click on Change Barline Joins
Click on the Violin ossia staff, Ctrl/Cmd-click on the Violin (non-ossia) staff, click on Change Barline Joins.
If you have Join with dashed barline selected (and applied) in Engraving options > Barlines > Ossias, it should appear like this.
