What is it called when two separate companies, for example two brokerage companies, work out of the same office and therefore are assumed as one compa

You did not provide sufficient information for me to understand what the real concern is.  If two people (or two companies) share the same office space, they ordinarily are just two people (or companies) sharing office space.  If, however, they adopt a name, or otherwise conduct their business in such a manner as to lead the public to think there is a business relationship between them, they may have inadvertently formed a partnership.  If you have engaged in a business transaction to your disadvantage thinking there was such a business relationship, then there may be liability on their part (either separately or jointly).  In order to prove your case, it may be necessary for you to prove to the satisfaction of a court that they held themselves out as being partners in business and that this somehow damaged you.  All this can be complicated.  If you think you have been damaged in some way, you should arrange a conference with a good business lawyer in your area.  A modest conference fee can give you considerable comfort in just knowing whether you have been damaged somehow and whether you have a reasonable remedy.