Police said the stash reached Spain hidden in packets posted from Hong Kong, adding that the British pair, aged 36 and 37, had been remanded in custody.
They did not specify when the two had been intercepted but authorities last month launched an investigation into drug trafficking via Alicante airport.
Their inquiries led them to a bar popular with Britons at El Campello, a resort just outside Alicante and where they made one arrest.
The second arrest followed some days later at the airport. Police, who valued the street value of the 12.2 kilo (26 pounds) stash at €3.5 million ($3.2 million), said the second suspect was seeking "to flee justice" when he was tracked down.
Police said examination of the drugs suggested "a strong synthetic cannabinoid" whose effects on the body were "80 times stronger than that of one gramme of traditional cannabis."
A police spokesman said it was consumed by first dissolving it in water before spraying a quantity on a cigarette.