Hi Patrick,
there's not a way to do exactly what you want. You can't hide the adjoining barlines in a grand staff instrument. You can do a workaround, though this will not give you an adjoining end of system line (but will at the beginning): add two basic staffs from score setup, then make one treble and one bass. Hide/delete the instrument names in score setup. Go back to the score, lasso select with the mouse over both staves, right click, and make a bracket group