That assumes, of course,there was a "plume". ( I've gotten a lot fonder of the plume in recent months.)
I'm not assuming a plume of any particular size or anything. Just the fact that the American Media building and at least one postal facility got contaminated indicate that handling anthrax in the form it was in the anthrax letters is going to spread it around.
Weaponized anthrax is insideous. It is designed to spread and infect as widely as possible. It really isn't plausible that two individuals were somehow targeted. The letters addressed to other individuals contaminated whole buildings. The odds that they were the only two affected by some otherwise unknown contamination or release of anthrax are astronomically against.
Odds are very strong that others were infected. They were either misdiagnosed, or the cases were covered up.