How to change just one voice in unisono voices?

Hi! I have 2 voices in one staff. Voice 1 has eight notes triplets, voice 2 quarter note triplets. Some of these notes are unisono (that means: there is a g1-eight note at the same time as a g1-quarter note – look at the attachement). I would like to move the quarter note g1 but I´m not able to select just this single note, I always move both the quarter note and the eight note (voice 1 and voice 2). How can I do this?
Bildschirmfoto 2017-03-27 um 12.55.41.png

Click the stem of the note you want to change.