Merging staves

I'm doing some some SATB choral work. I have separate staves for Soprano, Alto, Tenor, and Bass, as well as several instruments.

I have decided to merge the SA parts into one staff, and the TB into another. I tried using the recommended method: change the alto part from 1st to 2nd layer, cut the alto part and paste it to the soprano staff (whose notes are on layer 1). As I understand it, that should result in a staff with both parts. But when I do the paste operation, the soprano part is replaced by rests. What am I doing wrong?