I have. My randonneur bike now has a B&M cygo something premium something something, and it's pretty cool. It has a mechanical switch with three settings: on, off, and automatic. The light it produces is pretty impressive, but annoyingly uneven --a bright rectangle where I'll be riding in a few seconds, an excellent cutoff at the top, and a random scatter around the rest of the field area. SO I have it paired with a cheap battery light of the kind I describe, with an electronic switch. I did something to override the switch (IIRC, I put something on the button so it's constantly being pushed) and plugged that into the dymano as well. So now when i start rolling, the B&M comes on; when I get up to a certain speed, the additional light comes on, and evens out the uneven light from the B&M.

Necessary? No. In fact I'm not sure it gives me more light, since whether there's one light or two, they use the same dynamo and presumably the same amount of power.