The headlights are on individual circuits of their own, though they have several dual-circuited components in common. If there is an open in the circuit, the dimmer switch is definitely a suspect. Don't hit me for asking this, but have you actually tested for 12 volts at each headlight while on low beam? More than once, I have had customers to present a car with both low beams not working, and the problem was two burned out bulbs. Many folks just don't notice when they have ONE low beam not working, so aren't alerted to the problem until the second one burns out, then they think there is a big electrical malfunction, otherwise why would 2 lights go out at the same time? (When actually they didn't go out at the same time.) I have seen this happen to people who were no fools. Anyway, if the bulbs are ok, then yes, the dimmer would be a prime (but not the only) suspect.

sounds like bad dimmer switch

Check for a floating ground on your car. Or a Bad Ground. Make sure that between the hot side of your light and ground of the car(not the light). of your lights you get the proper voltages. Check the switch by having someone help you. Put your meter into the socket of the light and have someone turn on the DIM and BRIGHT lights see what voltages you get for each position. This will tell you whether the switch is working or not.