There is/was trim pieces that separated the two colors. It looks very similar to vinyl top trim, but is thinner. I can't say if anyone makes that trim any more. The two-tone cars usually had color combinations that were all the rage in the 60's but were no longer in style when the first re-paint came along.

The trim tag shows the second color. If I recall there's a dash between the two color codes.

I can't remember seeing a two-tone Firebird in person, either, but I have seen pictures of a dealership where there were two in the picture. The two had odd color roofs which were not standard vinyl top colors so they stood out.