Can we define two main methods in an application ?

No, we cannot.

Defining multiple entry point will give you a compile time error.

For example :-

class Program1
{
static void Main(string[] args)
{
}
}

class Program2
{
static void Main(string[] args)
{
}
}

Compiling the above code will give you an error :-

ConsoleApplication1.exe' has more than one entry point defined