I recall that one of the books of 4th Edition described two pairs of magic boots, who formed a group set together conferring bonuses to the wearers if both sets are worn. They had something to do with agility or jumping, but that's all I remember. And I cannot seem to remember which book this was: do any of you people know if this is an actual group magic item, and if so from which book?
I think you might be thinking of Rash and Reckless, an artifact that is a pair of boots from DMG 2.
Perhaps you're thinking of the Feystride Boots in Adventurer's Vault 2?
I didn't do an exhaustive search, but it looks like Item Sets (and Group Item Sets) were introduced in the Adventurer's Vault 2, and I don't see any of them in the other books I checked. The only boots on any of these group sets are the Feystride Boots.
The group set benefit for three wielders (ideally at least one Eladrin and at least one Elf or Drow) gives a +1 item bonus to speed.