Concatenate two char arrays?

If I have two char arrays like so:

char one[200];
char two[200];

And I then want to make a third which concatenates these how could I do it?

I have tried:

char three[400];
strcpy(three, one);
strcat(three, two);

But this doesn't seem to work. It does if one and two are setup like this:

char *one = "data";
char *two = "more data";

Anyone got any idea how to fix this?

Thanks