PAL-XFEL plans to begin double-bunch operation in 2027, with two electron bunches separated by 25 ns. The existing beam position monitor (BPM) electronics were designed for single-bunch operation, so the two signals overlap and bias the position measurement. To support this mode without modifying the electronics, we developed an external radio-frequency (RF) switch-based bunch selector that routes only the selected bunch to the BPM input. We built a prototype and conducted laboratory and on-site tests. This paper presents the hardware and experimental results.