I disagree (and I’m not the only one - see e.g. Choral divisi best practice? - #2 by dspreadbury). Condensing isn’t a good fit for choral music yet - too many rough edges e.g. where underlay isn’t identical between parts. If you need to switch between open score and short score, divisi can be useful, though it too has rough edges (e.g. vertical adjustment of lyric lines on lower divisi staves).
The main benefits of condensing are being able to show short score in one layout and separate staves in part layouts. This is rarely relevant in choral music - singers generally expect to sing from some sort of score, not from individual part layouts.
Back to the original question:
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Go to Write > Note Input Options and set this option to Allocate Equally (this is likely the missing step). Apply and Close.
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Select the music on the top staff.
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Copy.
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Select the first rest on the top staff.
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Shift-click the first rest on the second staff.
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Edit > Paste Special > Explode.
In some cases it may make more sense to cut than to copy. If the only thing present is notes and you don’t have Chord mode turned on, copying will work fine, though.