The attack happened at around 9pm on Sunday while headline acts Justin Timberlake and Jay-Z were performing in front of tens of thousands of people.
One man in his 20s is in a critical condition and the other man, also believed to be in his 20s, is in a stable condition, a police spokesman said.
No arrests have been made.
The three-day festival at the Queen Elizabeth park on the Olympic site came to a close shortly before midnight.
COMMENT RULES: Comments that are judged to be defamatory, abusive or in bad taste are not acceptable and contributors who consistently fall below certain criteria will be permanently blacklisted. The moderator will not enter into debate with individual contributors and the moderator’s decision is final. It is Belfast Telegraph policy to close comments on court cases, tribunals and active legal investigations. We may also close comments on articles which are being targeted for abuse. Problems with commenting? email@example.com