The freak accident took place about two hours into the sold-out concert last night.
Mirror members Edan Lui and Anson Lo were accompanied by around a dozen dancers on stage when a giant screen came crashing down.
It landed on top of a dancer, before tipping to the side and hitting another.
The horrific moment was widely documented, as many fans who paid top dollar for a ticket were filming the performance from their seats.
Some screamed in panic, while others were shocked or bewildered over what had just happened.
The lights went out briefly before the boy band's manager and veteran producer Wong Wai-kwan appeared on stage and apologised.
She also pleaded with the audience to leave so staff could focus on handling the emergency.
Thousands of concertgoers slowly made their way out of the Coliseum, many visibly distraught.
First responders arrived 10 minutes later and rushed the two dancers to Queen Elizabeth Hospital.
Top executives of MakerVille, the entertainment subsidiary of PCCW, arrived at the intensive care unit after midnight.
Lo Ting-fai, the agency's boss who used to run ViuTV, apologised once again and pledged to get to the bottom of the matter.
He revealed that one of the dancers, surnamed Li and better known as Mo, was conscious and able to communicate with doctors upon arrival.
Mo suffered a spinal fracture and remains in serious condition after undergoing surgery.
The other dancer, surnamed Cheung, escaped with abrasions and was discharged this afternoon.