Do they stack? The tooltip says one of them is Unique Passive the other one is Unique aura.
They do stack with each other.
The Unique Passive/Unique Aura part means that they don't stack with other copies of the same item - that is, if your team has two Abyssal Scepters, the enemies' magic resistance isn't reduced by 40, but only 20.
The general rule is that Unique Passives from different items do stack, unless the two passives have the same name (such as "Eyes of Pain" on Liandry's Torment and Haunting Guise). If the passive is unnamed than you can assume it will stack with other unnamed passives from a different item.