A theoretical study of the intermolecular double proton transfer in the adenine-uracil base pair has been performed to model the double proton transfer in the adenine-thymine dimer. The mechanism is analyzed in terms of the reaction force profile, which indicates that the activation of the transfer occurs via structural rearrangements to bring the interacting molecules close to each other to let the donor and acceptor atoms in the right position to achieve the transfer. It is found that only when the first proton transfer is partially completed does the second proton get activated, thus illustrating the asynchronous nature of the double proton-transfer process in base pair systems.