Rhox Faithmender

• WOTC Official Reminder Textofficial rules
  Lifelink — Damage dealt by this creature also causes you to gain that much life.

• WWizards of the Coast2012-07-01
  Official Ruling
  If an effect sets your life total to a specific number, and that number is higher than your current life total, the effect will cause you to gain life equal to the difference. Rhox Faithmender will then double that number. For example, if you have 3 life and an effect says that your life total "becomes 10," your life total will actually become 17.
• WWizards of the Coast2012-07-01
  Official Ruling
  If you control two Rhox Faithmenders, life you gain will be multiplied by four. Three Rhox Faithmenders will multiply any life gain by eight, and so on.
• WWizards of the Coast2012-07-01
  Official Ruling
  In a Two-Headed Giant game, only Rhox Faithmender's controller is affected by it. If that player's teammate gains life, Rhox Faithmender will have no effect, even when that life gain is applied to the shared team life total.

Rulings via Scryfall