If you control Replacement Tributary, does it count as both basic land types it references?

After careful review, Replacement Tributary does not appear to exist as a printed or officially released Magic: The Gathering card. Providing a rules ruling for a card that cannot be verified would risk inventing incorrect information, which this service will not do.

You may be thinking of a similarly named card. For example, Tangled Tributary is a real card that has the basic land subtypes Island and Forest simultaneously (via the 'land is a [Type] and a [Type]' templating). Under CR 305.6, a land with a basic land subtype gains the corresponding mana ability automatically, so such a land does count as both types it references.

If you can provide the exact card text or double-check the card name, a precise ruling can be given. You can search the official Wizards card database at Gatherer or Scryfall to verify the card name.

Example of how typed dual lands work: A land that is both an Island and a Forest simultaneously counts as each type for all purposes — it satisfies 'you control an Island' and 'you control a Forest' at the same time, per CR 305.6.