How did the plan make a war on 2 fronts even more likely?
Schlieffen took it for granted that if Russia (East) attacked Germany, France (West) would help it. Even if the French didn't help the Russian, the plan says to attach France first so they would have to divide their army anyway. He also didn't consider the possibility of Russia mobilizing their army in secret.
How did the French plan work?
What problems arose from this plan?
Belgium was protected by Britain. It left the eastern side of Germany unprotected and the Russians mobilized their forces faster than expected. It gave France easy access to Alsace and Lorraine. France was stronger than expected.
In case of war, Germany would attack France by sending soldiers through Belgium at a very high speed. They guessed that it would take Russia 6 weeks to mobilize their army and in those 6 weeks they would defeat France by taking Paris. After they finish on the west side, they would go to the East (Russia). They would then defeat the Russian who have just finished mobilizing their army because of how bad their railways and roads.