Usually a person has only one employer. However, there are times when someone is the employee of more than one entity.
Often this works to the benefit of the multiple employers. For instance, if an employee is injured on the job, worker's compensation is the exclusive remedy against the employer, and any recovery is capped. If there are joint employers, then the remedy against all is worker's compensation.
Again, like the employee v. independent contractor debate, the agreement of any parties as to the exact relationship does not determine the outcome.
But when this is coupled with that debate, it can work to the joint employee's advantage. Namely, all the employers would owe the various duties an employer owes to that employee. This would be crucial if there were an issue of one insolvent employer.
If you have such an issue, please consult with me. Employment law can be a maze of laws, and you need expert guidance to navigate it.
See Taylor v. Ford Motor Co., 944 N.E.2d 78 (Ind.Ct.App. 2011) for a case with three joint employers, and the employee being limited to worker's compensation. This case did so in spite of an agreement that the employee was only employed by one of the three.