How to have two notes played in the same time but having two different durations ?
You have to use the Voices system appeared in Guitar Pro 5. It takes place of the beat duration / note duration of earlier versions. Staves of Guitar Pro 5 contain two independent voices. This notation is closer to the musical notation standards. To edit a voice, choose the Edit > Voices > Edit Voice 1 [Lead] or Edit > Voices > Edit Voice 2 [Bass] menus. For more obviousness, you can choose to grey out the voice you’re not editing at the moment: Edit > Voices > Gray Inactive Voice. You can also use the Note > Sound Duration = X%. If you change the coefficient, the display of the note won’t change on the score but you will hear the difference in the sustain.