There is no connection between Philip Seymour Hoffman and Dustin Hoffman. The fact they share a last name is pure coincidence.
They are the only 2 best actor award winners to share the same last name though, so that's a link!
Dustin Hoffman on the other hand is involved in several projects that are currently in post-production (including Kung Fun Panda 3, where he is providing the voice of Shifu).
Looking into it, there are actually a pretty big number of actors and actresses who share the last name Hoffman.
You can find some of these famous Hoffmans on lucywho.com:
