From the C# spec v4.0, section 1.6.6:
The signature of a method must be unique in the class in which the method is declared. The signature of a method consists of the name of the method, the number of type parameters and the number, modifiers, and types of its parameters. The signature of a method does not include the return type.
You have two methods with the same signature because they differ only in the return type. This is not allowed.