Hull KR will lose Michael Dobson and Micky Paea at the end of the season.
Tongan forward Paea will cross the city to join Hull FC on a two-year deal after rejecting a new contract with Rovers.
And captain Dobson is moving to the NRL after Newcastle Knights activated a release-clause in the Australian's contract allowing him to return home.
Rovers chief executive Neil Hudgell said: "Michael has always harboured an ambition to go back and play in the NRL again and he goes with our blessing and best wishes.
"He has been a terrific servant to Hull KR and is probably the stand-out player for this club during the Super League era.
"Michael has also been an inspirational skipper this year and possess a never-say-die attitude which has probably seen him play in some games when he could quite easily have sat it out."
Paea, meanwhile, said: "I have enjoyed my time in England and I'm settled in Hull so that made a tough decision a little easier but it has been difficult with a number of options on the table.
"I have been grateful for the opportunity at Hull KR but I felt after two years with the club I needed a fresh start and, after weighing up my options, Hull FC was the best fit for me and my wife.
"I have a lot of respect for Hull KR and their supporters and I will give my all to the club for the remainder of the year and look forward to joining up with Hull FC in the close season."
A third leading Rovers player, Cory Paterson, has asked to be released from his contract early on compassionate grounds.