I know there are cards like Gemhide Sliver which let slivers tap for mana. Convoke lets all creatures do this once. Paradise mantle gives a creature this ability. But these examples only let the creature(s) tap for one mana.
Is there a permanent that lets creature/creatures tap for two mana rather than one?
I know there are creatures that can tap for 2 or more mana (like Incubation Druid and Gyre Engineer) but I want to give a creature the ability rather than have a creature with the ability.
A few cards do allow creatures to be tapped for 2 mana, each with its own restrictions:
Grand Architect has the ability "Tap an untapped blue creature you control: Add {C}{C}. Spend this mana only to cast artifact spells or activate abilities of artifacts."
Joraga Treespeaker has the ability "Elves you control have '{T}: Add {G}{G}.'" as long as it has at least 5 level counters.
Sachi, Daughter of Seshiro has the ability "Shamans you control have '{T}: Add {G}{G}.'"
Murgatroid99 gave excellent answers for what you specifically asked for. However, another route is to increase the mana generated by the mana source. Say for example, your creature is equipped with Paradise Mantle (there are lots of ways for creatures to generate mana. That's just one.) Then
Nyxbloom Ancient will cause it to generate 3X mana.
Leyline of Abundance will cause it to generate an additional {G}.
Mana Reflection will cause it generate 2X mana.
Kinnan, Bonder Prodigy will cause it to generate an additional mana.
Alternatively, you could turn the creature into a land, and use any of a number of techniques for increasing the mana generated by a land.
You could combine Kinnan, Bonder Prodigy with standard mana-dorks, or turn all your creatures into mana-dorks using Song of Freyalise. Mana Reflection works with creatures, too.
