Ms Cassie started to go to the lunchroom in one of the rooms in the back of the building but was met by Tarapata, who had blood on his hands and was carrying a knife.
Tarapata said, "Where is he?" and told his partner to return to the front of the shop, Ms Chang said.
Tarapata then returned to the back of the shop and went into several rooms. "He was looking for something, or someone," the prosecutor said.
Evidence from the scene presented to the jury included blood in several different parts of the building besides where the men's bodies were found.
Ms Chang said one or both men had been stabbed by the time the call for help was made and that Tarapata's attack was in two stages. "He must have returned to finish what he had started."
Video camera evidence shows Tarapata was inside the building for 2 mins 41 secs.
"They didn't see it coming," said Ms Chang, who told the jury it was a case of jealous paranoia. Tarapata had wrongly thought his partner was having affairs with the two men and several others, including the accused's father and best friend.
Tarapata has admitted killing the men but has pleaded not guilty on the grounds that he was insane at the time.
In a short opening statement, defence lawyer Bruce Hesketh said Tarapata was suffering a disease of the mind and had a psychotic incident as a result of schizophrenia.
Tarapata had told psychiatrists he had killed the men because God had told him to.
The Crown alleges that although Tarapata was not thinking straight, he was not insane. Cannabis and methamphetamine could have been influences.
"It wasn't rational, it certainly wasn't justified, but he knew what he was doing and he knew what he was doing was wrong," Ms Chang said.
She pointed to his calm demeanour and actions afterwards, which included hiding and trying to destroy evidence.
When he was arrested soon after he had asked for a lawyer and declined to give a statement. Ms Chang said this indicated he had made a considered decision about his rights.
Tarapata spent three months at the Mason Clinic but is now on remand in jail.
The Crown will call 28 witnesses in the three-week trial before Justice Pamela Andrews and a jury of six men and six women.