Check out his YouTube channel. I know the tools exist. Some people are just better at using them.
I don't mean to make this 'art form' any less enjoyable for you, but lots of software out there can take the vocals out of music tracks and change their speed or key.
The only other thing needed to make this is noting that the chord structure for these two songs is the same. And lots of songs have similar chord structures. There are thousands of songs based off the same chord structure as Canon in D, for instance, like all of these: https://www.youtube.com/watch?v=oOlDewpCfZQ
Discussion
Will do, sounds fun.
Here's an older interview where he details his process a bit. I'm sure he's upgraded to better tech by now.
https://www.stagepass.com/blogs/stagepass/mashups-bill-mcclintock