Could the allies have beaten germany without a "second front" in europe?

That's hard to say, but if Germany decided not to fight a two-front war, World War ll definitely would have continued for a longer period of time. Russia and Germany signed a Non-Aggression Pact in which they promised never to invade one another, but Hitler went back on his promise and sent troops to attack Russia. Big mistake. Despite studying Napoleon and his defeat in Russia, Hitler still decided to invade and was caught off guard by the Russian winter. Germany lost thousands of soldiers to illness and frostbite and exhausted all of their supplies and to top that off Germany suffered a huge loss at the Battle of Stalingrad and surrendered in 1943. If these events hadn't occurred, the probability of the Axis Powers winning WWll would have been much greater, or the Allied may not have won by such a large margin.

Each mans success depends on his own hard work