Backscatter communication offers a cost- and energy-efficient means for IoT sensor data exchange. The IoT vision for ubiquitous interconnection, in practice, demands multi-hop connectivity for robust and scalable sensor networks, as well as compatibility with such prevailing wireless technologies as WiFi. Today's backscatter solutions however typically follow a single-hop paradigm, i.e., tags do not relay for each other. This paper presents X-Tandem, a multi-hop backscatter system that works with commodity WiFi devices. For the first time, we demonstrate that sensing tags can not only work as relays for each other but also modulate their sensing data into a single backscatter packet, which remains a legit WiFi packet that can be decoded with any commercial WiFi NICs. We discuss the design details of X-Tandem and have built a prototype with FPGAs and off-the-shelf WiFi devices. The prototype demonstrates a two-hop implementation, achieving a throughput up to 200 bps with tag-to-tag distances up to 0.4 m and communication ranges up to 8 m. Compared to single-hop solutions, X-Tandem can improve backscatter throughput by more than 10x in challenging indoor environments with obstacles.