Welcome to the forum @pfv2!
There are a couple of ways.
The first one I would try is to use the Reduce function:
- Copy all the music from the S and A staves
- Select the staff that you want to be the SA staff
- Edit or Right-click > Paste Special > Reduce
(I’m not sure what would happen with differing lyrics using this method)
Another method would be to just paste each staff’s material into different voices. I’d start with the S part:
- Copy and paste the S staff into the SA staff.
- Flip the lyrics so that they are above the staff (select them and press F)
- Copy the A staff > select the SA staff > Edit > Paste Special > Paste Into Voice > New Down-stem Voice
Since you pasted the A staff after the S staff, it’s lyrics can just stay underneath.
Do either of these help?