Nowadays both of them cant, knuckles can use the master emerald, but there are multiple answers, in the comic series knuckles can use the emeralds like in "sonic and knuckles" for the genesis, except updated like now. Now tails, tails had a super form with the backwards compatibility of "Sonic 3" and "Sonic and Knuckles" when आप got the chaos emeralds in both games, but, When the emeralds became super, when that happened Sonic and Knuckles would have a Hyper form while tails had just a super form, look it up hes actually pretty cool he was glowing yellow with Flicky's flying around him that would attack and grab items as आप moved.
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shizno is right , although tails and knuckles could use them in sonic 3 & knuckles in reent games they havnt used them , in sonic हीरोस sonic uses the emeralds to make tails and knuckles superpowered , but not in there super form.
