To sum up, one is "A", one is "Un" ("ha" and "heng" in chines) -> these refer to the specific sounds.
Not to nitpick, but...
It's probably better romanized as "a1" and "hong1." Although, sometimes, the exhale is pronunced "e1" as in "em1mi2tuo2fo2", i.e. "Buddha preserve us." I was bored so I looked up hong1 in my 辭源 (Etymological Dictionary) [Beijing, 商务印书馆, 1996 (sorry, don't know the English name), 1996]and this is what it said:
Which I'm translating as: Many Sanskrit scriptures use the word hong1. This character is one of [the first?] 17 characters used by Buddhist esoteric sects [alt. secret religions/shingon] and secret speech.
My Chinese pretty much sucks so if someone has a better translation, please correct me.