You need to learn how pointers differ from arrays; you are writing over memory you did NOT allocate.
Also learn to indent your code!
Tim S.
Code:
#include <stdio.h>
#include <string.h>
/* Needed because I use a compiler written in this century */
void clrscr (void) {};
/* end of obsolete functions */
int main (void) {
char str1[12] = "ilikeyou";
char str2[12] = ".com";
clrscr ();
printf ("your two lines are \n %s \n %s", str1,str2);
strncat (str1,str2,2);
printf ("\nYour new two lines are \n %s \n %s", str1,str2);
getchar ();
return 0;
}