Check out the definition of a Workplace (art2) and it includes the common areas so the employers (RP’s) are responsible, under the RR(FS)O, for the whole of the building, not the landlord. They must ensure that any duty imposed by articles 8 to 22 or by regulations made under article 24 is complied with (art 5.1). Therefore they must co-operate with the other responsible person’s concerned so far as is necessary to enable them to comply with the requirements and prohibitions imposed on them by or under the RR(FS)O (art 22). The RP's must Co-operate and co-ordinate no choice.
You could make an argument that the landlord could be a RP if he/she has a workplace in the building (boiler room) and would be included with the rest of the RP’s. Also the landlord could be a Person Having Control (PHC) under art 5 (3), if for instance he/she was responsible for the maintenance of the fire alarm or FFE in the common areas and therefore be subject to art 13.