So this is the source of so many difficulties during my masters project. But there's nothing like putting yourself in at the deep end, blindfolded, upside-down, hands and legs bound and trying to work out how to swim.
Essentially the Layered texture node is the kingpin - outlined in red (with its input animcurvesTU node for each textures alpha, where the transition is keyframed). Feeding into this are both shaders that need layering. Theres a 'rocky' texture highlighted with gold with bump, with a rock displacement map. Then theres a 'skin' texture highlighted in blue, a misss fast skin shader, with multiple input files, a bump map and also a displacement map.
Colour and bump maps for the rocky and skin shaders are highlighted in the gold and blue boxes in the middle.
Both rocky and skin displacement maps are within the gold and blue boxes on the left. Displacement cannot feed into the layered texture node. Instead they must attach to a plus minus node which calculates the average displacement readings given both to then and passes that onto the Displacement shader / surface shader.
To further complicate things each displacements maps displacement values relate to their colour settings: Colour Gain, Colour Offset and Alpha Gain. (the GoZ application automatically calculated the 100% values for me). Those values need to be keyframed into the composition depending on when the transition or blend is supposed to occur. So when I wanted a 50% blend of each on frame 100 lets say, I worked out half the displacement values for each of these parameters and keyframed them in.
The two displacement maps of course had to be for the same original geometry. So I had modelled the head, and then created in zbrush a blend shape of it as the rocky version. The low-poly versions of each were brought into Maya and blend shaped.
The blend shapes had to have the same 50% value at frame 100.
Anyway so the combined displacement surface shader, and the layered texture node (for colour and bump) both connect to what I called "Bothshader" circled in green, and this is what I applied to the head meshes.
So then send this off to render, and the size of the maps, of course has a massive impact on render times. I got there eventually with much tweaking - but I wish I knew how much displacement added to render times before I set out. Once I had come so far down the road there was no turning back.
So the results are ok. But I would do it very differently given a second chance.