In modern studio situations, these types of vocal lines are going to be recorded separately (unless it's a big choir in a room).

If it were me, and I knew the vocalists couldn't read notation, I'd give them the lyrics with isolated mixes for each melody line, with a loud piano sound playing those lines. Melody A on Piano with backing, Melody B on piano with backing. Then they just need to match up the words with the rhythm of each melody line when recording. The engineer simply sticks those tracks together in post.

I'd also give them a rough demo of me singing it myself, each track / melody line (together and separate). Even if my singing is not great, they'll learn it quickly from hearing a demo. These sorts of sessions often move so fast that they'll quickly memorize it by ear, do a few takes, and move on to the next thing.

-Carl.