Federico Alzamora Previtali

Hello Anthony, 

I'm not sure what do yo mean by imposing multiple BCs on a single face but this error message occurs because a face or domain cannot be used for more than one "Named Selection" within a Fluent mesh, as Fluent is using the "Named Selections" to define the cell zones and boundary faces, respectively.
I would suggest giving the face a single Named Selection and assigning the BC that you want in Fluent.