Only one staff for two players

I started a new score and imported a part for 7-string guitar. Now I want to add another guitar and player to the score so I can have two parts being played at the same time.
This is not the same as adding a staff for an existing player, yet that is all I can find information about how to do.
How is this done?

Select the score layout in setup mode and tick the second players checkbox.

The screenshot shows check boxes for the players. They don’t appear on my screen.
I did see them once and it worked as you describe. But now they are gone again. No checkboxes to click.

Did you select the layout?
They disappear when you select a player