A 47-year-old man suffered stab wounds to his side and chest, while the other staff member was allegedly punched in the face.
Emergency services were called to the supermarket and the 47-year-old man was rushed to Westmead Hospital, where he remains after undergoing surgery.
Police seized a knife at the scene, which has been sent for forensic examination.
A 31-year-old man and a 21-year-old woman were arrested within minutes of the incident.
The man has been charged with wounding a person with intent to cause grievous bodily harm, while the woman was charged with assault occasioning actual bodily harm, and larceny.
The man, who is from Fairfield East, and the woman, from Wetherill Park, were refused bail overnight and will appear in Blacktown Local Court today.