One book is intended to promote social action rather directly while the other cannot be said to do so at all. While the commentary on issues of racial biases in Huck Finn are certainly part of the novel, they can only accurately be understood as minor elements of the narrative. This is not a book about slavery or social action. It is a book about conformity.
The bravery required to act as an individual connects these two works, however, and there is quite a bit to say about how these books can be connected.
I actually teach these two books back to back, starting with Douglass. Douglass's story is very moving because it is honest and not didactic. Twain's is compelling because of the loveable characters and the adventure. These are two different methods to achieve the same thing.
"Adventures of Huckleberry Finn" is a fictional story of a white boy who helps his friend Jim escape slavery. It is told from Huck's point of view, so the reader is not exposed to Jim's inner thoughts or feelings. Huck undergoes a character change when he rejects what society has taught him (that slavery is right) and decides to help Jim, no matter what the cost.
"The Narrative of the Life of Frederick Douglass" is a nonfiction account of Frederick Douglass' life as a slave and his subsequent escape from slavery. The reader is exposed to Mr. Douglass' thoughts and feelings because Douglass is the author.
Both books ultimately have the same argument, that slavery is an inhumane institution, but one does it through a fictional account that is often humorous and entertaining, and told from the point of view of a white person, whereas the other is a more serious account of a real person's story. It is up to the reader to decide which one drives the argument home better.