The men were attacked by a gang with knives at the Skellow Grange Boxing Club in Doncaster on Thursday evening (June 17).
Emergency services were called to the club at 6.53pm and the two men were rushed to hospital where they remain.
Detective Chief Inspector Nikki Leach, who is leading South Yorkshire Police's investigation said: "I understand that this incident will have caused serious concern for the Skellow community. We often talk of ‘targeted attacks’, which we are certain this was, but that doesn’t take away the fear that this kind of episode creates.
"Let me assure you that we have several positive lines of enquiry and my team, along with resources from across the force, are progressing this investigation at pace.
“We will not tolerate this level of violence in our communities and those responsible will be found and brought to justice.
“If you have information, it’s vital that you come forward and tell somebody what you know."
Anyone with information cal call police on 101 quoting incident 838 of June 17, or alternatively, call Crimestoppers anonymously on 0800 555 111.