Enter 2nd voice where it exists only for half of the measure

Enter a 2nd voice in the whole measure, then delete it in the first half.

Click on an empty part of the measure, press N then [Ctrl][Alt]2
(you are now in Note Entry, 2nd Voice)
Enter a half-note rest and then your notes
Make the rest in the first half of the bar invisible by clicking on it and pressing V

I prefer to make rests invisible rather than deleting them since it makes later corrections to the score easier and i find it easier to read. They don't show up on the printed score and you can hide/unhide all invisible marks on screen.