Hi,
current version of tuxguitar don't have multi-voice support ( but it's planned for future versions )
So you can't do exactly that now.
But you can write same composition on a single voice.
* set the first beat on 4th (without dot)
* set the second beat on 8th instead of 4th
To heard the sound exactly as on that tablature, you should add some tied notes. ( "4th + dot" is same as "4th tied with 8th" )
To apply it to the previous example, you can add 2 notes on second beat, and tied with the first ( original dotted ).
and on the third beat, you should tied a note with previous 8th note ( to que the original 4th sound )