Compile-time error: Multiple definition of 'main'

You cannot have two main functions in the same project. Put them in separate projects or rename one of the functions and call it from the other main function.

You can never have more than one main() function in your project since it is the entrypoint, no matter what the parameter list is like.

You can however have multiple declarations of other functions as long as the parameter list is different (function overloading).

File 1

#include <iostream>

using namespace std;

int main()
{
    cout<<"Hello World";
    otherFunction();
    return 0;
}

File 2

#include <iostream>

using namespace std;

void otherFunction()
{
    cout<<"Demo Program";
}

Dont forget the appropiate #includes.

You can't have two main functions. In fact you can't have any two functions having the same signature through out your project (not your files).
And as Mr.TAMER said main is a special case, you can't even have two functions called main.