Insert minim with a quaver duplet and crochet (sorted)

I'm attempting to insert a minim with a quaver duplet and crochet notes in the second half of a 4/4 measure on the same stave. However every time I try to add the other part changes the initial note(s) inserted.

Example is the treble clef in the 19th measure of Summer Swing by John DeHolt.

TIA for any assistance from a newbie on MuseScore.
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Hang on a minute, just found something about voices, I'll post back shortly. Selecting the green 2 for the second voice is not changing my note cursor to green. Using MuseScore (1.3 rev 5702).
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Sorted by closing MuseScore and opening again, and got the measure sorted. Helped by the video from http://www.musescoretips.com/2011/08/how-to-write-two-parts-on-one-stav… to understand what I should be doing and what I was not seeing. Helped also by http://musescore.org/en/handbook/voices