They came out at about the same time, were both really big at about the same time, and there was lots of press linking them together. Maybe you had to be there, but I personally always think of the albums as a pair.
Nirvana - Nevermind / Pearl Jam - Ten
The two big albums as grunge got popular. When I think of one, I always think of the other.
Wilco - A.M. / Sun Volt - Trace
The demise of Uncle Tupelo produced these two bands who released their debut albums at about the same time (many mags reviewed them as a pair)