When a group of users or devices move together, network mobility resolves how to manage the location transition of the whole members. Two kinds of solutions have been proposed at different layers. Network Mobility (NEMO) Basic Support Protocol (BSP) is the extension of Mobile IPv6 at the network layer. On the other hand, Session Initiation Protocol (SIP) based network mobility (SIP-NEMO) is defined at the application layer. However, on the perspective of ubiquitous network environment, a mobile router requires to possess several egress paths to the Internet for providing uninterrupted services and aggregating good enough bandwidth. Multihoming in network mobility support becomes one urgent and nessceary issue. Since Mobile IPv6 based NEMO BSP, called MIPv6-NEMO hereafter for convenience, uses a bi-directional tunnel between home agent and mobile router, Multihomed MIPv6-NEMO would suffer and deepen header overhead and suboptimal routing. In the past, we have proposed SIP-NEMO to achieve route optimization. In this paper, we investigate and analyze how to extend the current SIP-NEMO to support multihoming further. The proposed multihomed SIP-NEMO is simulated in different cases and integrated with the IEEE 802.21 Media Independent Handover (MIH) standard. According to the simulation result, the SIP-NEMO can keep route optimization no matter which multihomed case is configured.