The problem with the gauges is that they are both LED lit. So just throwing a Potentiometer in there to dim them doesn't work. I did find a company that makes a specific LED dimmer...
http://www.autometer.com/cat_accessoriesdetail.aspx?vid=185
Not ideal. Hard to mount and expensive. I think each one would need its own dimmer because the LEDs are most likely different. Also I don't know how many LEDs are in each one. I would guess one but who knows. And, there is the problem that its an autometer part working with a B&M and StreetGlo gauge. I would think it would work but again who knows?
I may just throw a kill switch on them at some point. They only bother me on really dark roads. Driving in the city doesn't matter because the outside lights are so bright.
I was hoping someone out there made a gauge cover that was tinted or smoked that would fit right over the gauge. I have searched a ton on the web and in parts stores and found nothing like that.