To say it in words, each topic has many posts and each post belongs to a topic - that is exactly what relationships you need.
When you have defined them, as described in the documentation, you can do something like this:
$topics = ORM::factory("topic")->find_all();
foreach ($topics as $topic) {
// $topic->topicname
foreach ($topic->posts as $post) {
// $post->posttitle
}
}