In Ontario, the common law's common employer doctrine has been considered in several wrongful dismissal cases.
... it seems clear that for purposes of a wrongful dismissal claim, an individual may be held to be an employee of more than one corporation in a related group of corporations. One must find evidence of an intention to create an employer/employee relationship between the individual and the respective corporations within the group.
In Downtown Eatery Ltd. v. Ontario, an employee sued his "paymaster company" for wrongful dismissal. Following a trial, he was awarded substantial damages. However, as it turned out, the defendant company had no assets. As a result, the employee had to take a subsequent action against all the related companies and the two main principals of all the companies in an effort to widen his net of potential sources of recovery.
If you have been wrongfully dismissed, you should consult a lawyer who can consider with you whether you were employed by more than one company for the purposes of determing the contractual and fiduciary obligations which are owed by the employer under both the Employment Standards Act and at common law.