Double cluster

To notate a cluster of chords as two clusters (as on the 4th beat above) do the following:

Add extra note heads (secondary notes) to existing notes with <insert> (can also be done using chord mode)
Untie the added secondary note with <'>; or the untie button from the note properties menu ( ) from the buttonbar.
reverse stem direction with <ctrl-> / <ctrl->
connect the single notes with flags to form a new cluster with <,> or link clusters in the note attributes menu.

A completely different way of achieving more or less the same thing is through a new part. By notating both parts (with different stem direction) on one staff using the score overview you can more or less get the same thing done. If (as in four-part choral music in two staves) the two parts are independent, that is probably the clearest.

See also