There are many variables that could come into play with this situation, for example, who controls the work, whose employees are exposed, and who creates the hazards. OSHA does clarify this somewhat in its Compliance Directive Multi-Employer Citation Policy, which should be consulted. (Generally speaking, OSHA holds the employer of the employees exposed to the hazard ultimately responsible. But, as noted in the Directive, in some cases, more than one employer may have responsibilities.)