I know there were artists that released two different albums on the same day, Bruce Springsteen, notably. I guess Guns n Roses did with the two Use Your Illusion albums too.
But I can't think of a band that released their first two albums within, say, a week of each other, especially since the seventies (Artists sometimes would release three or even four albums a year in the 50's and 60's- they usually sucked of course
)