To create a vertical line spanning multiple staves, select a note in the voice on the top staff, then use Ctrl+click (Windows) or Command-click (Mac) to select a note in the voice on the bottom staff, then click the desired line type. The line will be created to enclose the chords on the selected staves.
I can’t get this to work. If I select one note on the top staff and one note on the bottom staff, I only get a line spanning the notes on the top staff.
all of the instruments must belong to the same player.
Actually, not. All staves must belong to the same instrument. Different instruments even in the same player do not allow to span, only spanning possible across the staves created with edit/staff menu. It is a pity. Still no solution for the Syncro point in multi-instrumental score except graphical one.