Two parties SPDZ Protocol Input sharing
The MAC key is not pre-owned by someone but generated in the offline phase. Each party $P_i$ chooses a random value $\Delta_i$ and the sum $\Delta = \sum \Delta_i$ is the MAC key. To ensure $\Delta_i$ is consistently used in all shares, ciphertexts of $\Delta_i$ are exchanged and zero-knowledge proofs are used to prove they are correct. 

See for example the protocol in Fig 4 in the Overdrive paper.