Before you can really think about what your title and your subtitle should be, you need to decide what you are going to say in your paper. Your title will need to, in some way, be based on what you argue in the essay. This is true even if your essay is not argumentative or persuasive. You are going to be focusing on some particular aspect of the act and your title should reflect that.
For example, you might focus on the ways in which the act was unfair. If you do that, your title might be something along the lines of “American Injustice” and your subtitle could be “How the Fugitive Slave Act was unfair to African Americans. “ On the other hand, you might be focusing on the reaction to the Act and how that helped to cause the Civil War. In that case, your title could be something like “Another Step Towards War” and your subtitle could be “How Reactions to the Fugitive Slave Act Drove the North and the South Farther Apart.”
It would be a good idea to write your paper first and then decide what to title it based on what you have said in the paper.