The Twin Rings of Wonder

This set of dual magical rings requires each ring to be worn by a separate person. They have no power if attempted to use singularly. It is thought that a single user attempting to use both rings the same time can cause a dangerous magical feedback effect resulting in some unpleasent side effects. 

When pressed together and the correct incantation is spoken by both wearers, the rings bestow a separate magical power to each welder.

The most famous set of rings is known to allow one user to poly-morph into any known animal (HD or Level limits are recommended); while the second user takes on the form of a water-elemental.

IMAGE SOURCE

The wearer of the water-elemental ring is able to chose to take on a purely liquid, a gaseous vapour or a solid form made of ice (an ice bucket, for example).

The welder of each ring must state what form they are choosing as part of the incantation and the form may not be altered once the effect has taken place (you chose to be a pterodactyl, you're a pterodactyl until you chose to transform back, or the effect wears off). The rings are useable only once per day and the effect lasts up to one hour.

Other Wonder Rings pairs with different effects are thought to exist, but their powers are as yet, unknown.