First of all, you have to create some dynamic memory space. Then you can just strcat the two strings into it. Or you can use the c++ "string" class. The old-school C way:
char* catString = malloc(strlen(one)+strlen(two)+1);
strcpy(catString, one);
strcat(catString, two);
// use the string then delete it when you're done.
free(catString);
New C++ way
std::string three(one);
three += two;