Exec. Summary: Whether single or dual, do yourself a favour, and get one.
I generally ride on city & country roads (usually 2 lane, minimum - or no shoulder, with moderate to busy traffic). Around where I live, cars & trucks tend to ignore the speed limits, and with each approaching vehicle from behind, I used to have chills running down my spine. Yes, I was doing the over/under shoulder checks, but as you know, those are just "snapshots" - the vehicle might swerve right - left, speed up, etc. the moment you look away. So I got a Garmin Varia Radar, to have some peace of mind. Now I could see when a vehicle was approaching, and how fast it was relative to me. But I still lacked major info: Was it a car - a truck, was it going to pass me very closely, would there be enough room for me to escape right, were there other vehicles just behind it (a couple of times, very close travelling cars registered as one car in my radar). Then I found about Trieye.
From the first couple of meters of my first ride with it, I was blown away by the practicality and simpleness of the (left side, single) mirror. There it was, just like the field of vision of a car's side mirrors, I was able to see what was going on behind me. Garmin was "beeping", and with a simple head move, I could see what was coming, and could position myself accordingly. Since that day, I never left for a ride without Trieye. The left sided single mirror gives a lot of info, but at some fast right turning descents, it gives you only limited vision due to the curvature. I was thinking - if there was another mirror on the right side, this issue could be mitigated - and then dual sided lenses came out. I immediately got one, and have no regrets at all, now having full view of my back, either on straight roads, or curvy roads.
Some friends asked me if the mirrors interfered with the front field of vision - answer is, 98% of the time, no. For the remaining 2%, it is not the front end vision, but, a small section of the left & right - bottom parts, which you can easily cover with a simple move of your head. So yes, I absolutely recommend them.
