Back in the 1950s, 1960s, and 1970s, many music artists release more than one album in a year. I have no idea how it was like but that's normal in music industry back then. Then 1980s came and onwards, artists usually release at least one album within 12 months, or even years.
There are artists, though, releasing an album with two CDs. They may be called double albums. While others release two albums as Part 1 and 2, either simultaneously or within the same year. Recently, Taylor Swift just released her new album "evermore", which is a follow up to "folklore". The latter is was released just less than 5 months ago. Swift even claimed "evermore" is the sister album of "folklore" which comes to show they're sequels.
Ariana Grande also released her album "Thank You, Next" just less than 6 months after "Sweetener". Whether the two albums are Parts 1 and 2, the similarites are there, and both albums has upside down album covers. The two albums were number 1 sellers.
One of the most notable two-part album is Justin Timberlake's "The 20/20 Experience" and "The 20/20 Experience – 2 of 2" in 2013. The two albums are released in the same year, and 7 months apart. Both went to number 1, though the latter's sales is underwhelming compared to the first part, which is one of the biggest sellers that year.
Nelly released two albums in a week with "Sweat" and "Suit", but they seemed different musically. "Suit" was number 1 then, while "Sweat" is at number 2. The most notable among, I think, is Guns N Roses back in 1991 with "Use Your Illusion I" and "Use Your Illusion II". Both released at the same week, and both are big sellers.