This paper presents an efficient and reliable protocol that enables a pipelined transmission using two channels in wireless sensor networks. Nodes in the network form a tree originating from a sink node. A sharable slot is allocated to each tree level and one unique channel is assigned to every other level in the tree. Data transmission is performed from the lowest level to the highest level, allowing two simultaneous transmissions. The level-order data transmission using a sharable slot also increases the reliability of data reception greatly because every node has multiple chances of receiving the same data while it reduces competition for data transmission. Using the simulator Cooja on Contiki OS, we showed that the proposed approach far outperformed the Deluge protocol in terms of completion time and control overhead.