The Greek word for united in this verse is a term from the field of biology and means “parts united so as to form a single part,” such as when two branches of a tree fuse together. The first half of the verse reinforces the truth that we have been baptized into His death (Romans 6:3). Ponder that for a moment. Being united with Jesus means that we participated in His death (Galatians 2:20).
The second half of the verse explains that it logically follows then that we are united with Jesus in His resurrection (also see Romans 6:8). God sees us as being on the resurrection side of the grave! May our walk be propelled by the joy of that knowledge.