Guitar Pro supports more than one "voice" for each track.
Each Guitar Pro staff contains two independent voices in the case of GP5, and 4 for GP6. This type of notation is thus closer to the standards of musical notation.
To edit a voice, choose the "Edit > Voices > Edit Voice 1" or "Edit > Voices > Edit Voice 2" menus (or voice 3 or 4 in GP6).
For more clarity in Guitar Pro 5, you can choose to grey out the voice you're not editing at the moment: "Edit > Voices > Gray Inactive Voice".
In Guitar Pro 6, simply choose to work on Voice 1, 2, 3, or 4, or in multitrack mode.