Airlines sometimes use a chasing agent. Since handling agents are usually quite large with many different departments, a chasing agent is employed to act as a management company to act as station reps.
A company such as wickes services at gatwick and stansted look after the interests of a number of airlines to ensure the smooth running of the whole turnround including check-in.
OR
Another answer could simply be that HandlingAgent 1 is subbing the work out to HandlingAgent 2 because of faulty or insufficient equipment/manpower. So you may see H.A.1 loading bags and H.A.2 providing push back.