GP6 - How can I type in two notes played at the same time but with different duration values?

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.