There is no built-in 2x EXP multiplier in the original FireRed/LeafGreen. No cheat code toggles it natively, and no in-game item (not even the Exp. Share) doubles total gains.
Pokémon defeated in trainer battles always grant 50% more XP than wild Pokémon. pokemon fire red exp multiplier x2
Pokémon received from another trainer (via trade) gain 50% more experience in every battle. Lucky Egg (1.5x Boost): There is no built-in 2x EXP multiplier in
While there is no single "x2" item, you can stack specific conditions to exceed that value naturally. pokemon fire red exp multiplier x2
Pokémon FireRed cheats: Full list of cheat codes and how to use them