The phrase 'two decades before' is correct and can be used in written English.
For example: 'The company had experienced significant success two decades before.'.
Two decades before, they were in the same gang.
Well, nearly two decades before serving as governor.
McKeon served in Congress for more than two decades before deciding not seek reelection in 2014.
More than two decades before, Salazar was taking steps to become a priest in his hometown, Los Angeles.
The only real noticeable change was that the scantily-clad girls I recall from two decades before hadn't aged.
In 1954, two decades before Watergate, Mr. St.
These amateur videos publicized bin Laden's cause through informal channels two decades before YouTube.
It was an achievement that no one had thought possible two decades before.
It would be another two decades before it won another seat.
Mr. Traficant represented the Youngstown area for nearly two decades before his 2002 conviction.
Never mind that the war happened more than two decades before he was born.
