The Mightstone and Weakstone

The mana value of a melded permanent is the sum of the mana values of its front faces. A creature that becomes a copy of a melded permanent has only the characteristics of that combined back face, and its mana value is 0.

When a pair of cards are melded, the result is a single creature that's represented by two cards. If the melded creature dies, both cards are put into your graveyard. As it leaves the battlefield, both of those cards are turned face up again. If the cards are put on the top or bottom of your library, you choose their relative order.

A player prompted to name a card may name the combined back face, and each player has the right to know that combined back face's characteristics at all times.

Only two cards belonging to the same meld pair can be melded. Tokens, cards that aren't meld cards, or meld cards that don't form a meld pair can't be melded. If an effect instructs a player to meld cards that can't be melded, those cards remain in exile.

While a meld card is in any zone other than the battlefield, it has only the characteristics of its front face. The same is true while it's on the battlefield with its front face up.

Note that the permanent represented by the combined back faces has a color indicator.