The relay for the exterior lights is at the far back of the main fuse box, have to take off a few screws to get to it. This controls all the outputs to the exterior lamps except the center stoplamp, which is hardwired from the stop lamp switch. i believe it would take a double failure of the relay to cause this to happen (both lowbeams out). I would tend to stick with the combi-switch diagnosis.
ps it's not an extremely expensive part to replace or hard to do. I think the relay costs alot more for the part, but of course is easier to install. I think the combi switch is in the $100-$200 range and can be done in 1 or 2 hours