PRC Controller described a potential conflict event when Local Control failed to note an aircraft stopping on the runway with traffic on short final; listing staffing and combined positions as causal factors.

Narrative

I was working Ground Control and there was training taking place at Local 1. The locals were combined because we did not have the staffing to de-combine and still train. About 10 minutes prior to the incident; I noted that traffic complexity and volume were to the point where the Supervisor should have opened the Local 2 position. I was being relieved at the Ground Control position and during the briefing the person who was relieving me happened to look at Runway 21R. The relieving Controller observed a Cessna stopped about 1;000 FT down Runway 21R and aircraft X on short final. The person relieving me alerted the trainer right away and they were able to send aircraft X around without incident. This could have been very ugly had the relieving Ground Controller not noticed the problem on Runway 21R. The Supervisor was in the cab and watching the traffic; but was not aware of the problem on Runway 21R. Recommendation; staffing; staffing; staffing! The Supervisor should not reduce safety when busy by keeping the locals combined in order to accomplish training.