RNO Controller experienced a loss of separation event while working Approach Control in the Tower Cab utilizing the D-Brite equipment; noting lack of procedures covering this type of operation was a causal factor in the incident.

Narrative

I was working RADAR in the Tower Cab on a D-Brite scope; which is smaller than our RADAR scopes in the RADAR room. When we are departing Runway 16; all IFR departures depart straight out on a heading of 165. Air Carrier X was inbound from the south on a course that parallel the departure course by about 6 miles. Ten miles from the airport; I cleared Air Carrier X for a visual approach Runway 25. Working in the Tower; there is no type of run-down for departures. Right after I cleared Air Carrier X for the visual approach; Air Carrier Y tagged up off the departure end. I could not restrict the altitude of Air Carrier Y because of the MVAs. At that time I noticed Air Carrier X turning west toward Air Carrier Y and descending rapidly. This was a very unusual and very unexpected maneuver for a visual approach to Runway 25. I could not turn Air Carrier X because of MVAs. I attempted to establish visual separation from the aircraft; but was unable. I was able to look out the windows and I had both aircraft in sight prior to the aircraft getting within 3 miles of each other. There was also a controller change happening at the Local 1 position at the time of the event. Recommendation; perhaps; when RADAR is being worked in the Tower; some sort of notification needs to be made to the RADAR Controller before an IFR aircraft is departed. Something; as simple as passing the flight strip prior to departing an IFR aircraft could prevent this from happening again. In the future; when working RADAR in the Tower; I will try to pay more attention to the Tower's traffic to see when an IFR aircraft is departing. I will also protect more for departures since I can't always keep track of Tower traffic.