Both aircraft were approaching from the SSW and flying assigned headings. Neither aircraft had been assigned an altitude. Just prior to impact, approach control (A/C) told the C-402B to fly 360 to follow traffic and asked what his altitude was. When the pilot responded he was at 2300 ft, A/C told him "You've got a Cherokee right there with you." The collision occurred about 5 seconds later. The PA-28 made a forced landing in a retention pond. The C-402B landed without further incident. Of the 8 ATC personnel on duty in the TRACON, only 3 were assigned control positions. The remainder were either on break or out for dinner. The accident controller had 9 aircraft under his control at the time. Both aircraft were within his control and had been assigned transponder codes, radar identified, and assigned headings for sequencing. The only traffic advisory he issued either aircraft about the other was when the collision was imminent and he told the C-402B that the Cherokee was right there with him.