I can't say what might have been special about the year 1974, in this regard. But it does seem that a lot of pop music artists were able to release two top-notch albums in the same calendar year, throughout the Sixties and Seventies.
Granted, nearly all artists turned out the work at a higher rate back in the Sixties and early Seventies. The Beatles released their first six studio albums in the UK at a rate of two per year. Indeed, from what I've read, it was kind of a cause for concern, to the record-buying public, when 1966 came and went with only one new Beatles album. (Bob Dylan was the subject of similar concern, that same year, when he didn't follow up
Blonde on Blonde until the following year ... most of the public apparently being unaware of his motorcycle accident.) Still, that doesn't account for the inspiration level being so high, among so many artists.
The Jimi Hendrix Experience -
Are You Experienced and
Axis: Bold As Love (1967)
Led Zeppelin -
Led Zeppelin and
Led Zeppelin II (1969)
Elton John -
Elton John and
Tumbleweed Connection (1970)
Alice Cooper -
Love It to Death and
Killer (1971)
Stevie Wonder -
Music of My Mind and
Talking Book (1972)
Roxy Music -
For Your Pleasure and
Stranded (1973)
Be Bop Deluxe -
Sunburst Finish and
Modern Music (1975)
David Bowie -
Low and
Heroes (1977)
Cheap Trick -
Cheap Trick and
In Color (1977)
The Ramones -
Ramones Leave Home and
Rocket to Russia (1977)
Joe Jackson -
Look Sharp! and
I'm the Man (1979)
Gary Numan -
Replicas and
The Pleasure Principle (1979)
I'm sure there are others; these are just the ones I'm spotting on my shelf.
Of course, the validity of the list above depends on your perspective -- whether, as Reno once said, I'm applying the term "classic" to albums whose merit and significance has stood the test of time, or just albums that I like a lot.