Lamback finished second in the men’s 400m freestyle (S7) with a time of 4:56.46.
On Wednesday, Lamback set a world record and won the gold medal in the men’s 100m backstroke (S7).
Lamback now has three medals this week: one gold and two silvers.
“It was competitive. My coach said to go out there and get after it, and so that’s what I did," Lamback said. "It was me and Dave Roberts the whole way. He just had a little more left than me at the end. I had an incredible race and am happy to be three for three.”