Every so often, a mashup of songs come out and it doesn't quite work. It's nice in theory, mixing two songs together, but the need for the two tempos to be similar usually means it can't be done. Unless you got the two artists to join together to make a new mashup like when Ed Sheeran joined Passenger, it's not really going to work.
Well there is one other option.
You can get someone with a ridiculously good voice to sing both sections of the song into one glorious mashup. That's what Evynne Hollens did, and despite her hard to spell name, she absolutely nails it. Damn.
When you get a mashup done well it's poetry in motion. Probably more poetic than the original musical poem, or "song". If you like mashups, here's the aforementioned Ed Sheeran and Passenger:
https://www.youtube.com/watch?v=F2K_7-MHt3k