With the objective to let a listener compare two BTE hearing aids placed on the same ear and passing the sound through the same earmould, a new device has been developed. It replaces the ordinary plastic tubing connecting the earhook of a hearing aid to the earmould and has the ability to switch earmould tubing between two earhooks. The binaural device is completely worn on the head and connected to a control computer by a cable. Acoustical measurements were done to see if there was any difference between the device and ordinary tubing and if the two pathways were acoustically equal. The damping of the device through a closed valve was also investigated as well as the influence of the proximity to the head when two BTEs were placed together on the same ear. Both coupler and real ear measurements were done with the finding that there was a difference in the high frequency region between the device and ordinary tubing. The two pathways were equal within 2 dB and the damping was frequency dependent (>40 dB, 1000 Hz). The results from the real ear measurements showed that the proximity effect was within +/- 1.5 dB.