I have one complete headset and one single headset. Of course, I put one on the charging station and use the other. There is no other way. Unless you take batteries and change them each time.

Two Sennheiser "HDR 175 single headphones" can interfere with each other if they are used in close proximity to each other. Here are some reasons and solutions: - Interference due to wireless signal transmission: Wireless devices can interfere with each other, especially if they are placed in close proximity to each other. It is advisable to place transmitters at least 50 cm away from other wireless devices to avoid interference. - Changing the installation location: Try to change the installation location of the transmitters or headphones to minimise interference. By taking these measures, you can reduce the likelihood of interference between the two headphones.

If you mean can you pair an old 165 with the new 175 base station, then yes. The other way round, I don't know, but I assume so because the wireless protocol in this case has probably remained the same.