The attack happened at around 9.30 pm in the Donegall Road on Saturday night.
One of the males sustained a serious head injury.
He was initially described as being in a critical condition in hospital.
However, the Belfast Trust has said this morning that his condition is now stable.
Investigating officer Detective Sergeant Ken Leckey said: “The second male sustained facial injuries which are not believed to be life threatening.
“The assault occurred whilst a band parade was taking place in the area.”
However, the attack is not believed to be sectarian.
Sergeant Leckey added, “It is understood that the two males are brothers and I would appeal to anyone who was in this area and witnessed the assault to contact police on the non-emergency number 101.
“Or alternatively contact Crimestoppers anonymously on 0800 555 111.”