In music theory, a voice is a succession of single notes or chords following a rhythmic sequence.

In Flat, to have two voices on the same staff, use the voice tool. Click the Voice button on the left toolbar, then choose Voice 2. Press v as a keyboard shortcut.
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Working with voices on Flat for Education
Split Staff
When a measure contains two voices, use Split Staff to give each player their own part.
Swap voices
Select a range of notes, click the voice button, then click Swap voices.