/*На вход программы поступают имена трех
файловНеобходимо записать содержимое
первых двух файлов в третий в вперемешку*/
/* program to merge two files
and stores their contents in another file*/

#include <stdio.h>
#include <stdlib.h>

int main()
{
   FILE *fs1, *fs2, *ft;

   char ch, firstFile[20], secondFile[20], mergedFile[20];

   printf("Enter name of first file\n");
   gets(fileFile);

   printf("Enter name of second file\n");
   gets(secondFile);

   printf("Enter name of file which will store contents of two files\n");
   gets(mergedFile);

   fs1 = fopen(firstFile,"r");
   fs2 = fopen(secondFile,"r");

   if( fs1 == NULL || fs2 == NULL )
   {
      perror("Error ");
      printf("Press any key to exit...\n");
      getch();
      exit(EXIT_FAILURE);
   }

   ft = fopen(mergedFile,"w");

   if( ft == NULL )
   {
      perror("Error ");
      printf("Press any key to exit...\n");
      exit(EXIT_FAILURE);
   }

   while( ( ch = fgetc(fs1) ) != EOF )
      fputc(ch,ft);

   while( ( ch = fgetc(fs2) ) != EOF )
      fputc(ch,ft);

   printf("\n\nTwo files were merged into %s file successfully.\n",mergedFile);

   fclose(fs1);
   fclose(fs2);
   fclose(ft);

   return 0;
}

output

Enter name of first file
firstFile.txt
Enter name of second file
secondFile.txt
Enter name of file which will store contents of two files
mergedFile.txt
Two files were merged into mergedFile.txt successfully