Mongoose

This is a bad misuse of terminology.

"Stacking" refers to the ability to multiply a buff over and over, up to a certain limit. For example, warriors sunder armor stacks up to 5 times, and mages fire vulnerability from scorch stacks up to 5 times.

This buff does NOT stack. If it procs again while the 15s buff is up, then the buff is RENEWED to 15s, but the effect does not multiply.

What I think people are talking about is that it can be active once per weapon, so if you have two weapons with it on, then it can be active as two separate buffs with separate timers. This is not what most people think of when you say "it stacks".

This is more or less exactly like Crusader. It will stack if used on both weapons, but won't stack if proc is from same source.

It won't work for ranged as all weapon enchants on meele weapons require a meele hit to proc.

No, it's not a bad use of terminology.

Even this website, http://dictionary.reference.com/browse/stack clearly shows that;
"Stack" is used to determine the ability of an item to pile-up upon itself.
If you have two mongooses and both proc and give you 4% attack speed and 240 agility then yes, they are indeed stacking.

If they didn't stack, they would only provide 120 agility and 2% attack speed increase even if both procced at the same time.

Do not think that just because the two different enchants have two different buff icons that they do not "stack".
If one triggers within 15 seconds of the other you will certainly gain the effect twice over for a total 240 agility and 4% attack speed bonus, regardless of the fact that they use two different buff slots.

The only reason they don't share the same buff slot on your character is because blizzard didn't want a mongoose proccing and refreshing the existing buff from your first mongoose.
If that happened, you could quite often have 120 agility and 2% speed increase for 14 seconds and then 240 agility and 4% attack speed increase for a further 15 seconds if your secondary mongoose procced during the last second of your first mongooses' proc.