while ( !fin.eof() )
    {
        if ( !fin.eof() )
        {
            do
            {
                curr = next;
 
                fin >> next;
 
                fout1 << curr << endl;
            }
            while ( !fin.eof() && (curr <= next) );
        }
 
        if ( !fin.eof() )
        {
            do
            {
                curr = next;
 
                fin >> next;
 
                fout2 << curr << endl;
            }
            while ( !fin.eof() && (curr <= next ) );
        }
    }

//attempt to read value from file into next
fin >> next;
 
//while success reading from file into next
while ( fin.good())    
{        
  do
  {
    //assign next to curr
    curr = next;
 
    //write curr to file #1
    fout1 << curr << endl;
 
    //read in next value from file
    fin >> next;
 
    //while able to read something into next and 
    //if prior last reading <= next reading
    }while (fin.good() && curr <= next);
  }
 
  //if something in next at end of first do/while loop
  if(fin.good())
  {         
    do
    {
      curr = next;
      fin >> next;
 
      //write curr into file #2
      fout2 << curr << endl;
    }while( fin.good() && curr <= next);
  }
}

Michael              Dillon
Classname         ClassMark
Teachername     TeacherSomething

Classname      Classmark
Michael           Dillon
Teachername  TeacherSomething
 
or
 
Michael           Classmark
Classname      Dillon
Teachername  TeacherSomething

//read file, storing information in two containers of your choice, say vectors, depending on column location in file.
int i = 0;
vector <string> v1;
vector <string> v2;
while(fin >> v1[i])
   fin >> v2[i++];
 
//ask user to select which column (which means which vector) they want to sort
cout << "enter 1 if you want to sort column 1 and two if you want to sort column 2" << endl;
int choice;
cin >> choice;
if(choice == 1)
   mergeSort(v1);  //sort container 1
else
   mergeSort(v2); //sort container 2