My contribution to Motorhead Month…
It might not have been the first-ever split, but in 1981, Motorhead teamed up with labelmates Girlschool for the St. Valentines Day Massacre EP. The former were the kings of fledgling NWOBHM label Bronze Records, having just put out Ace of Spades, while the latter was making waves as the first all-girl metal band after the release of their debut, Demolition. The early February release suited the album title perfectly, and the cover photo lives on in metal lore.