With all due respect, ConjureLady. Let me correct you as I myself apart from Santeria, have works in Palo. Palo is not the "darker" side of Santeria and the religion itself has nothing to do with Santeria. Palo being named "The Darker Side of Santeria" is a clear misconception from ignorance and/or poor resources of study. Palo, clearly, is not a him but a religion; with various initiations and a clear belief system. Now why do so many people consider Palo to be "dark"? Simply because there is more free will, for example Nganga(The Cauldron that holds the deity)can be used either for good or for bad. Let's take a police officer with a gun, he can use it for the good of others OR he can use it for bad. It is the "Free will" of the person whether he/she uses it for good or bad. This has nothing to do with dark or light, it simply is.
Now that we got that out of the way, lets move forward. Many people can notice similarities between Palo and Santeria due to a similar belief system as well as the deities themselves. Are they the same? No, They are not. For example, Chango and Siete Rayos have many similarities due to their attitude and how they are viewed but they are not the same deity. You will notice these days that there are many Santero/a's crowned that are also scratched(initiated) in Palo for many reasons. Sometimes due to the fact they are requested by a Eggun(Spirit) in their spiritual court as well as they may be marked for it. Vice-versa for those that go into Palo as well may get crowned(initiated) in Santeria.
Palo is simply a deeper and more misunderstood religion due to many misconceptions Hollywood has put on it. Those that are interested in knowing about Palo, I recommend not to go to any website online and believe everything it tells you. Books? Good but no the best way. My opinion is the best way to learn about Palo is through those that are already initiated as they have more experience through the religion apart from those that have 0 experience in the religion and are learning from things they read online. Learn from those that are already initiated into the religion rather than those who learn off the internet.