At the end of WK, it sounds like Prince has one vocal singing, "Why did I waste my kisses on you *now*?" while another sings, "Why did I waste my kisses on you *love*?" Does anyone else hear him using both vocals
simultaneously? Also, has he done something similar in other songs.

Yes this is a technique he uses often. I can't think of any example right now but he does this a few times on TRC. I'll get back 2 u and let u know what he says.

I've heard him do that too, cant think of anything right now though

In "positivity", right at the bit where the song goes

'All the boys, and all the girls, U r the new KINGS of the world.....'

the backing vox just state a double

'all the boys'

instead of following the lead and phrasing just 1 'boy' and following up with the 2nd 'girl'.

Just one xample I happened 2 know right now since I've been listening 2 "positivity" in my car, on repeat 4 a few days.