Last update which has been quite a while ago added the "hidden" synergy between the two true-artifact daggers "Judgement" and "Mercy", meaning if you wield them both they will gain additional powers.
Aside from this documented "hidden synergy" there are a bunch more really hidden ones, here's the exhaustive list:
- Daggers Judgement + Mercy.
- Dual-wield *thanc daggers (Narthanc, Nimthanc, Dethanc).
- Moltor's boots and pick.
- Bilbo's Sting and lockpick.
- All three of Durin's arts: Ring, Helm, Axe. (Only effect: The Ring will then lose its experience-drain effect.)