We consider a two-way relay channel (TWRC), in which two source nodes exchange their data with the help of a relay node using network coding (NC). The sources, to be exchanged are double-layer bit-streams, can be partitioned into a high-priority (HP) bit-stream and a low-priority (LP) bit-stream. The HP bit-stream should be generally more protected than the LP bit-stream because it is assumed that the HP bit-stream includes more important data. Such an assumption is applicable to video communications using a base and enhancement layers of scalable video coding (SVC). In this paper, we propose an NC mapping rule that can continuously improve bit error rate (BER) performance even when the relay node is located anywhere between source nodes. We define the target and the minimum requirements for high-quality video communications and confirm that our proposed NC mapping rule remarkably improves the BER performance, regardless of the mobility.