I got them mixed up too, but in retrospect that might have been intentional on Asimov's part. Hardin ends up being the true intellectual heir of Seldon, after all.
I guess the reason in bugs me so much is because you get so little time with each character to have the 2 of them mixed up. It slows down my ability to read it and to understand what's going on.
It's really tricky business when you try to delve into naming conventions for a futuristic society, and doubly tricky to try and keep the names of different characters distinct yet consistent with each other. I'll say this, all of the characters felt like their names were from the same universe, and I liked that. Of course since the history spans such a vast period of time, I wonder if that consistency isn't actually a flaw.
Even half-way through the book, I still find myself having to stop and think about which person they are refering to when using the H & S initials.