Courtesy of a contact I have in the film industry:
Essentially, all black and white films since the very early days of film manufacture are “two-emulsion”, or more, films. In order to obtain films with speeds fast enough for indoor photography, films required multiple sized grains and for a film to become panchromatic, that meant that grains of different sizes would require dying to record red, green or blue light. Each of those dyed grain “sets”, in their own emulsion could then be mixed into one large emulsion to coat or split up to coat in several layers.
Another means of making a “two-emulsion” black and white film would be to split the grain sizes into small to mid-sized and mid-sized to large each set to its own emulsion, and mixing all three dyed silver grains together for a slow-to-mid and a mid-to-fast emulsion. All panchromatic black and white films manufactured for still film photography have been manufactured like this for a very long time, perhaps 40 years or more.
Why?
Separating the grain sizes gives the advantage for the film manufacture to better control the film speed when the emulsions are made, and also when they are coated. Light absorbing dyes can be added to optimize a specific region of the DlogE curve’s speed by dying the specific emulsion, which will lead to better consistency and linearity in the product. These are tools that the manufacturer would not have if all the speeds were coated “all as one”.
And yes, due to their physical characteristics, the emulsion components with different speeds are usually coated one on top of the other.
Thanks for the info on 120 - I don't recall ever using it in that format, but certainly used it in 127, 620 and 616.