I just could not tell how you knew this for sure. I thought maybe you thought the two names of the consuls indicated a plural source for the document.
No, the consulibus is just the date of the document.
I simply looked up the full text of where this comes from:
CTh.1.15.10
Imppp. gratianus, valentinianus et theodosius aaa. ad syagrium. vicario africae aditus provinciae proconsularis inhibendus est tantumque ei consilii gratia in thevestinam civitatem accessus pateat. canoni autem cogendo annonae praefectus immineat. vestes largitionales sinceritatis tuae cogat officium, cui negotio etiam rationalis insistat, ita tamen, ut principe loco apparitores tuos maneat et coactionis instantia et deceptionis invidia. vectigalia sane apud karthaginem constituta vicariae praefecturae apparitio procuret. pp. karthagine. dat. vii k. sept. auxonio et olybrio consul. (379 aug. 26.).
I took Imppp. to mean something like imperatores, and there are 3 names that are mentioned that appear to address a single vicar ... so if PP is an abbreviation that refers to people rather than some other terminus technicus, I would have taken it to refer to multiple people.
Actually, I just looked up those 3 names, as my historical knowledge about the late empire is rather limited, and it looks like those 3 people somehow reigned together as emperors. In that case I would take PP to just mean
patres patriae.