I always say that the Soviets defeated Nazi Germany and could have eventually won without a second front. But i don't know if they could've won without the threat of a second front. Regardless, less than a year after D-Day, Germany was in ruins.
its all conjecture there allready were 2 Fronts (Afrika and Russia) before D-Day so its hard to see if it would have had much difference - its almost universally assumed that Germany never could have crushed Russia without aid and Russia would have eventually pushed them out and then moved on to crush them but how long it would have taken is another thing - D-Day is considered more about Keeping Western Europe Free of soviets rather than free from Nazis nowadays...