Assume we have two computing parties for SPDZ protocol. The two parties are also the input parties. How can they share the MACs on their input shares assuming one party is the master who owns the MAC key and one is the client who do not have the key ?
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.