The Mightstone and Weakstone

• WWizards of the Coast2022-10-14
  Official Ruling
  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.
• WWizards of the Coast2022-10-14
  Official Ruling
  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.
• WWizards of the Coast2022-10-14
  Official Ruling
  In the Commander variant, a meld card's color identity is determined only by the mana costs and mana symbols in the rules text of its front face. No symbols or rules text of the permanent it melds into are considered.
• WWizards of the Coast2022-10-14
  Official Ruling
  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.
• WWizards of the Coast2022-10-14
  Official Ruling
  Note that the permanent represented by the combined back faces has a color indicator.
• WWizards of the Coast2022-10-14
  Official Ruling
  One card in each pair of meld cards has an ability that instructs you to exile the two cards and meld them. If you control more than one object with one of those names, you select one object with that name to exile.
• WWizards of the Coast2022-10-14
  Official Ruling
  While a melded permanent is on the battlefield, it has only the characteristics of its combined back face. Any effects that modify how the new object enters the battlefield will consider only the combined back face.
• WWizards of the Coast2022-10-14
  Official Ruling
  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.
• WWizards of the Coast2022-10-14
  Official Ruling
  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.
• WWizards of the Coast2022-10-14
  Official Ruling
  If an effect moves a melded permanent to a new zone and then affects "that card," it affects both cards.
• WWizards of the Coast2022-10-14
  Official Ruling
  When two cards are exiled and melded, they each leave the battlefield, then return together as one new object with no relation to either of the objects that left the battlefield. Counters, Auras, Equipment, and other effects that affected those two cards don't affect the melded permanent.

Rulings via Scryfall