C++ source files and headers

Question

If I have too many functions inside my main source file (main.cpp) and I want to put some functions into another source file (other.cpp), then include the headers in both main.cpp and other.cpp, is this better or putting everything in 1 main.cpp is enough? Provided class is forbidden in this project.

other.h
void myfunction();

other.cpp
#include "other.h"
void myfunction()
{
codes
}

main.cpp
#include "other.h"
int main()
{
myfunction();
}