Date: 201812
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Aircraft: Skyhawk 172/Cutlass 172
Operator: Personal
Operating under: Part 91
Flight plan: VFR
Flight phase: Landing
Airspace: Class D ZZZ
ATC / advisory: Tower ZZZ
Second aircraft: PA-28 Cherokee/Archer/Dakota/Pillan/Warrior
Reporter organization: Government
Reporter function: Trainee; Local
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Workload; Situational Awareness; Training / Qualification
Second reporter function: Supervisor / CIC
Anomaly: ATC Issue All Types; Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Detector: Person Air Traffic Control
When detected: Taxi
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: I was providing OJT on Local Control. Traffic was building with numerous aircraft in the north and south patterns. I asked the CIC to split the Local Control position. It took him a couple of minutes as he was working on an IFR release for us and attempted to take a briefing but it was too busy to brief. I told the Developmental working to make his pattern players full stop so that we could brief and start fresh. Aircraft X was cleared for a full stop landing on RYXXL; number one. Aircraft Y was cleared to land RYXXL; number two; following Aircraft X. Spacing was close; so the Developmental told Aircraft Y that 'S turns' south of course were approved for spacing; hoping that the pilot would make them and adequate spacing would be achieved. We moved on to other traffic and by the time my scan made it back to RYXXL Aircraft X had landed approaching a turn of south of the runway at Taxiway P; closely followed by Aircraft Y who had already landed and rolled out. I instructed Aircraft X to exit the runway immediately and Aircraft Y followed him off at taxiway P. I should have shut down the pattern and then reconvened once we were able to split the Local position. I also could have taken over for the Developmental at a point where we had decent control over the traffic; it had developed to a point that we were reacting and the situation was chaotic. I do have to mention that the staffing levels are low and would greatly benefit from more controllers so that we could have the Local Control position split.


Second narrative: At the time of the event; I was the CIC. Traffic was moderate to heavy with moderate complexity due to the mixture of aircraft in the traffic pattern with itinerant aircraft. There was training on Local Control only. The Local trainer asked to split local operations. I then told our Ground Control to make a split ATIS and I moved over to the local-north position to preview it. As I began to preview the position; I realized traffic was to heavy and complex to split at that very moment. I then instructed local to full stop all traffic and shut the pattern down so we could split positions. Local then began to tell aircraft to land and taxi back. Aircraft X was at a very slow taxi speed and exiting RY2XXL at Papa intersection when I noticed Aircraft Y rolling out behind Aircraft X; maybe within a few hundred feet. I believe the mixture of training on Local and what appeared to be training in both of the aircraft involved led to the loss of separation. Aircraft X appeared to be at a very slow taxi speed exiting the runway and Aircraft Y appeared to be at a faster than normal roll out for an aircraft cleared to land.I would recommend training be stopped if traffic level and complexity requires locals to be split.


Synopsis: Tower Controller and Controller In Charge reported a runway incursion after attempting to land all aircraft due to workload and splitting Local Control.