Adding an "alternate" right hand/vocal part.

So I'm not the greatest at terminology when it comes to sheet music, and I'm not really sure if a thing like this exists, but I'll do my best to explain haha.
So basically, the bit I'm working out has a left and right hand part, but there's also an accompanying vocal melody. It's kind of boring without this part, so I usually just sing it, and since it's literally only 16 measures - a teeny part of the song, I don't want to have a whole other track there for vocals (also the weird robot synthesized voice kind of gets on my nerves).
I was thinking I'd add a voice 2 for that section, but some of the notes are the same between each part and I wouldn't want it to be crowded/confusing.
I'm wondering if there's a way to add a third 'instrument' but only for those 16 measures, to avoid wasting space all the rest of the way, or maybe inputting the voice 2 on ledger lines but with an 8vb.
If this makes any sense and someone has a suggestion, any input would be helpful.