I have to agree with Ian. In the early days, they were sometimes coated as two layers. Using properly ballasted or specially compounded chemicals as I mentioned elsewhere prevents crosstalk. There may be a few examples nowdays that are two layer products come to think of it, but I have not investigated this. In one case I know of, the omission of a chemical causes one grade of contrast and inclusion of it gives the other grade emulsion. In another case, the ratio of bromide to chloride did the job.
When we visited Ilford we enjoyed a demonstration of their coating machine. I thought they explained that although the two emulsions were applied at almost the same time through a special applicator, they did not mix, but stayed quite separate on the paper. They certainly were applied from seperate tanks and not premixed. Maybe I was mistaken?
They could be mixed in the lines, but they could be dual coated using a slide or curtain coater Dave.
Kodak has used both mixed emulsions and dual layer (slide) coatings. I was not aware that Ilford used the slide method. I assumed that they mixed them in-line as, I assume, did Ian. All things are possible. I don't know what Polycontrast IV used. The early papers like Varigam and others used 2 layers. I was told that Kodak had begun using 1 layer. All things change. If it is wrong, I am sorry.
I don't know how I would do it yet if I hand coated a VC Azo type paper. I still have that in mind.
PE
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OK - I guess I'm living in the past...
I see on Ilford's site, they say "MULTIGRADE papers are coated with an emulsion
which is a mixture of three separate emulsions."
When we visited Ilford we enjoyed a demonstration of their coating machine. I thought they explained that although the two emulsions were applied at almost the same time through a special applicator, they did not mix, but stayed quite separate on the paper. They certainly were applied from seperate tanks and not premixed. Maybe I was mistaken?
Even if the paper is coated with one (mixed) emulsion, which is the case with variable contrast papers (Multigrade is an Ilford trademark), there is a protective supercoat layer and therefore the coater must have at least two solutions (called "melts") applied onto the paper support. In all modern coater designs, ALL layers of the coating materials are applied simultaneously. The commercial emulsions are fairly viscous and taken up by a very fast flowing paper so they maintain the multilayer structure. It is most surprising to cut a section of a color negative film material and look under transmission electron microscope (take a slice perpendicular to the film plane).
I know MG papers are typically made by laying down 2 layers of emulsion, one with high contrast and one with low contrast.
Is it possible to mix 2 emulsions, each with differing contrasts, and expect them to function as a MG paper?
Renwick and his people at Ilford made the first variable contrast paper emulsion. There were a couple of different generations of technologies in this, so you will probably find this subject very interesting to study.
The first generation of Renwick/Ilford used a bromide emulsion without a dye and chlorobromide emulsion with an ortho dye. The bromide emulsion is made about twice as fast as the chlorobromide emulsion. Due to the technical limitation of that time, higher contrast emulsion must be the chloride emulsion, thereby green light giving a higher contrast than blue light. Note that this is OPPOSITE to today's system. Another shortcomming of this system was that, when high contrast is obtained with a yellow filter, silver used in the bromide emulsion is completely wasted, because they are not exposed at all. Usually, higher contrast should utilize more silver content of the emulsion for the best results.
The next generation is a gross simplification from this. and suggested possible operating mechanisms. Potter and Hagaman found that a variable contrast emulsion can be made from one batch of emulsion by using considerably less amount of an ortho dye than needed to cover the surface of all grains. In such cases, the effect of the dye is most pronounced on coarse grains (fast and softer contrast), and thus green light will give softer contrast than blue, the latter which exposes all grains.
Later generations are based on the second generation filtering scheme, but may use multiple batches of the starting emulsions, each of which may be dye sensitized with different dyes with absorption peak that are significantly far. The dyes used in this application must be "non-diffusing" and the dye must be used very carefully, in an optimal quantity, which varies with the grain size (and therefore grain surface area per mole of silver).
In short, it is very useful learning process and stimulating mental exercise to understand how multicontrast emulsions work and how they were made, as well as the technical difficulties that the pioneers had to solve, but I don't think it's very fruitful to set a goal to reprecate them in your darkroom. It is FAR easier to make a range of paper emulsions from low contrast to high contrast.
Check out U.S. Patents 2,202,026 (Renwick), 2,280,300 (Potter et al), and, for your reference for the conceptual father of this technology, British Patent 15,054 of 1912 (Fischer).
Multigrade emulsions 
Linear Mode
