In
GuitarPro 5, you can add two voices to a staff. You have to switch to the appropriate voice before entering notes. The upper voice in GP5 is called "Lead" and the lower voice is called "Bass". There is a "Voices" item in the "Edit" menu and also buttons in the tool bar for switching between the two voices.
To add notes of different durations at the same point in a measure, they must be entered in different voices.