TUS Controller described a loss of separation event when a sweeper vehicle crossed a runway hold line without proper coordination; the reporter noting current guidelines referencing the shared use of Runway3/21 is unclear.

Narrative

Training was in progress on Local 1 and Local Assist was staffed. Runway 3/21 is shared by Local Controller and Ground Control. The portion from Taxiway A to Taxiway D1 is owned by Ground Control unless Local Control/Local Assistant calls 3/21 'hot' for landing traffic; or aircraft are taxied to 3/21 for departure. Runway 3/21 was not called hot and a Guard Sweeper called from the Guard Ramp; holding short of 3/21 at D2 requesting to cross 3/21 and sweep Taxiways A and D. I told the Sweeper to cross 3/21 and proceed as requested on Taxiways A and D; holding short of all runways. As soon as he started to move; Local Control 1 said 'where is the sweeper going?' I told the Sweeper to hold short of 3/21 and I saw him turn around and he read back the hold short instructions. Due to the angle from the Tower I could not tell if he crossed the hold short line. Shortly thereafter; I saw a Lear rolling at taxi speed down Runway 3. Since 3/21 was never called hot; I was not sure if the Lear had landed Runway 3; or Local Control 1 was taxiing him down Runway 3. Recommendation; other facilities have a runway which was shared by 2 Locals; but this situation where part of the runway is owned by Local; and part is owned by Ground; leads to confusion. Either the runway should belong to Local all the time and vehicles be required to use the perimeter road; or if Local Assist is staffed; they should be responsible for coordinating use of Runway 3/21 with Ground. I think Local Control 1 thought Local Assist had coordinated with Ground Control; or Local Assist thought Local Control 1 was going to due the coordination for training.