Mark Jones, 43, from Mountain Ash, was shot in a lay-by on New Road in an area known locally as Lletty Turner Bends.
A 46-year-old man was arrested on suspicion of attempted murder Tuesday evening and is being held in custody.
Police have been granted a further 36 hours to question another man arrested on Monday.
A second man has been arrested in connection with a shooting  in Rhondda Cynon Taff on Sunday.