As explained in other threads, both business entities could be in the same building but on different floors, and the scheme would still be illegal. They could be on the same floor at what looks like the same address but is technically two separate addresses, and the scheme would still be illegal.
The fact that it’s a common practice that (almost) everyone knows is happening doesn’t make it legal either.
So, what to do? If OP doesn’t need this job, and an LSA Art. 14 situation exists, ask someone to help with the writing of a nice, proper termination letter and terminate the (ahem) buxiban contract immediately. If the employer is the “what, you sent us a letter? no we have no idea what you’re talking about” type, get insurance by making it a legal deposit letter. 