Network mobility is proposed to let a mobile network change its point of attachment and still keep all nodes attached to the mobile network globally reachable. However, due to finite bandwidth, limited signaling coverage and frequent link failure, a mobile network needs multihoming to access the Internet simultaneously on the perspective of performance and reliability. In this paper, we extends our previous work, i.e., SIP-based network mobility (SIP-NEMO), to support multihoming. A mobile network is considered to possess more than one egress gateway. The proposed multihomed SIP-NEMO provides dynamic synchronization among different egress gateways. Once an egress gateway joins or leaves the mobile network, the multihomed SIP-NEMO can establish or switch sessions on another alternative egress gateway. According to the simulation results, the proposed multihomed SIP-NEMO can achieve route optimization for data transmission.