Prosecutor Wayne Forrest said Saturday that the second teen is a friend of the 16-year-old Bridgewater-Raritan High School student who was arrested on Thursday.
The second teen is identified as a 17-year-old Bridgewater resident who is a senior at Immaculata High School in Somerville. Neither teen's name has been released.
Forrest said police searched the 17-year-old's residence Friday night and discovered a number of rifles and shotguns, as well as shotgun shells. He said the guns are registered to the boy's father.
Both boys are in custody. The 17-year-old has been charged with possession of destructive devices.