To adjust the settings for duets double-click any of the duet events (Duet dialog box will appear) and select Duet settings button to open Duet settings dialog box.
Here you can define the text settings for Singer 1 and Singer 2 -- just select the mode on the top and adjust the settings. Please note that the number of lines apply to half of the screen only (Singer 1 uses the upper part and Singer 2 uses the lower part of TV screen). Settings for Both singers mode are copied from general video settings.