soPDF is here -- the GUI I wrote for it is
here. (I'm going to arrogantly assume that's the one you meant; thanks for liking it! Another GUI is available
here.) However, I don't really think of soPDF as a tool for splitting a duplex PDF into parts; the splitting options are meant for splitting a single page up so that it fits on a reader in landscape mode. Though I guess it's possible you might luck out and it would split a duplex one just where you wanted to as well.