If two people on my team have Abyssal Scepter does the debuff stack on enemy champions?

No, as the item clearly states "Unique". Unique items and properties never stack with identical ones against your enemies. Sometimes with teammates like Aegis and Stark's Fervor...but not against opponents.

You can have two starks buffs on you provided you own one of the starks. So 'never' is not the correct answer. I would guess not but I've never tested, sorry Mimus Cipher.

Clearly you are an ******* who thinks everyone else is stupid and can't read.

Aegis of the Legion stacks for the two people that have this "Unique" item smart ass know it all.

Starks and Ageis called bro.

Identical auras on different champions stack. for example...if Ashe and a Heimerdinger has aaq Abyssmal scepter each. then a nearby foe will lose 40 magic resist...due to -20 from Ashe's, and -20 from Heimerdinger.

PS: Why would an Ashe have an Abyssmal Scepter...I do not know.

Actually I'm not sure if this applies for any item other than Ageis and Starks.

If it does, I'm going to start doing Fiddlesticks/Katarina games where we both have abyssals.

That is false.

The Aura's only stack on allied champions IF they also have the same item. So if Sivir and Shaco both have Stark's and are next to each other they will get a second copy of the 20% lifesteal and 20% attack speed buff but the -20 Armor will only apply once to nearby enemies however.

Then if the rest of their team joins them (Morgana, Pantheon, and Alistar) with Pantheon also having a Starks then Pantheon, Sivir, and Shaco would all have 2 copies of the buff, while Alistar and Morgana have 1. Enemies still only have one copy of the debuff.

He asked about his enemies. Your Stark's do not stack against your enemies, they stack on you.

I dont think it does, it stacks only when you have the item and someone on your team gives you the aura too.
For example, If wto people on your tram have aegis, but you dont, you only have the benefit of one.
Im pretty sure enemies would receive 1 debuff only.