> It's either more difficult to implement this

Something like this should be enough

    "topiconesubtopictwo" [label = "subtopic two"];
    "topictwosubtopictwo" [label = "subtopic two"];

Or alternatively you can use a counter.

    12 [label = 'subtopic two'];
    22 [label = 'subtopic two'];

If you avoid spaces you can also forget the quotes (but quotes are safer)

     topiconesubtopictwo [label = "subtopic two"];

But avoid to name nodes with '_' and '.'

     topic_one_subtopic_two [label, etc...]
     topic.one.subtopic.two [label, etc...]