How about “Young Lust” vs. “Great Gig in the Sky” by Floyd? Or pretty much any song from the Syd era.
65daysofstatic - “retreat retreat” and “sister.” One is definitive post-rock, plus an uplifting anthem for the band. The other is a slow, foreboding ambient electronic piece. No shared instruments, themes, or genres between the two.
Smash Mouth - Pet Names
Smash Mouth - Whole Lotta Love
Or basically anything from Fush Yu Mang compared to their later stuff.
“Teardrops on My Guitar” and “Blank Space.”
Beach Boys: “Surfin USA” and “All I Wanna Do” (there are plenty of other examples too)