Complex sentence: The Rapids reached the final once before, in 1997, which it lost to D.C. United; they also reached the final of the Lamar Hunt U.S. Open Cup in 1999.
Split into simpler sentences: The Rapids reached the final once before, in 1997, which they lost to D.C. United. They also reached the final of the Lamar Hunt U.S. Open Cup in 1999.