tornados, how do they work?

Having a look at this imgur video fragment¹ and was reminded of an old question: what is it exactly that makes the funnel cloud visible? I had decided that there were at least two reasons, but this suggests a third one.

Specifically, compare these two stills from earlier and later in the video.

Early: Later:

The later image has a shape to it that I've seen relatively frequently. A funnel of cloud, which looks like it's made of the same sorts of water droplets one finds in sky clouds, grows from the sky down towards the ground. Another cylindrical cloud grows up from the ground towards the sky; this one seems to be made of debris picked up by the storm. The debris cloud frequently has a different (usually larger) radius than the vapor cloud. The two funnels may or may not meet or overlap.

Those are the two mechanisms I had worked out before now: the vapor cloud and the debris cloud. But early in the video, there seem to be two very different vapor clouds: one with a rounded profile, which seems to evolve fairly slow and have poorly-defined edges, and another with better-defined edges that evolves more quickly. In the screen capture above, the smaller funnel looks almost needle-like. What's up with that?

I've seen videos where the vapor cloud grows and shrinks quite rapidly. I think this means the humidity in the most intense part of the updraft is quite close to the saturation point, and small changes in the pressure can induce condensation over a pretty wide area. I have worked out previously that condensation should tend to increase updrafting, since (a) condensation releases heat, and (b) removing water vapor decreases the number density of air molecules. And I've decided previously that this feedback between condensation and updrafting (since condensation happens faster at lower pressure and therefore also at higher altitudes) is part of why cumulonimbus thunderstorms grow so tall and so violent. But I haven't considered the possibility that this same mechanism, taken to some extreme, drives the strong central updraft that we refer to as "the tornado."

So that's today's question.

Are there two different vapor funnels early in the formation of this storm? If so, why? If not, why does the early funnel have this two-tiered shape?

Looking a second time. The larger, softer funnel cloud seems in some frames to be hollow, with the better-defined cloud materializing in its center. Trying to get stills out of that fast time-lapse led me to a longer non-time-lapse video about the same storm, from the same photographer and including some of the same video. Here are some other observations:

At 00:47: a substantial debris cloud coming up from the ground, while the vapor cloud is still very short and stubby (and not even visible in this screenshot):

At 01:36: this is a better still image showing the "hollow outer funnel"; it's much clearer in the video.