Christ made only one sacrifice for sins, not two. That sacrifice was His death on the cross. Today LDS believe that the Atonement happened in Gethsemane because the early leaders of the church taught that Christ was a polygamist and that is the reason he was crucified.
"Who his own self bare our sins in his own body on the tree, that we, being dead to sins, should live unto righteousness: by whose stripes ye were healed" 1 Peter 2:24
"But God forbid that I should glory, save in the cross of our Lord Jesus Christ, by whom the world is crucified unto me, and I unto the world" Galatians 6:14
"And, having made peace through the blood of his cross, by him to reconcile all things unto himself; by him, I say, whether they be things in earth, or things in heaven" Colossians 1:20