1) Create custom IModel with IModelSimpleProperties with IModelUVLock class
	This implementation, for you, should probably be very simple, as it will just delegate to the two submodels.
	getTextures() - Return ImmutableList.of() (empty list)
	getDependencies() - Return an ImmutableList containing the RLs of the other two models
	getDefaultState() - Return some dummy value like ModelRotation.X0_Y0
	gui3d() - Store given value for later
	smoothLighting() - Ditto
	uvLock() - Ditto
	bake() - The complicated one
		Load the other two models with ModelLoaderRegistry.getModel
		For both of them, do some instanceof checks and call gui3d, smoothLighting, and uvLock on them with the stored values
		Two new IModels go into local vars
		Bake both models with the given IModelState, VertexFormat, and textureGetter
		Use IPerspectiveAwareModel.MapWrapper.getTransforms on the IModelState to get an ImmutableMap
		Create an instance of your custom IPerspectiveAwareModel containing the other two IBakedModels, the ImmutableMap, gui3d, smoothLighting, and uvLock and return it.

2) Create custom IPerspectiveAwareModel class
	Also very simple for you, as this thing is basically dispatches to the other two IBakedModels depending on a boolean expression.
	getOverrides() - Dummy out; probably return ItemOverrideList.NONE.
	isAmbientOcclusion() - Return the value of smoothLighting stored earlier.
	isGui3d() - Ditto but with the gui3d value
	isBuiltinRenderer() - false unless you want breakage
	getItemCameraTransforms() - Dummy out; probably return ItemCameraTransforms.DEFAULT.
	getQuads() - Dummy out with empty list or delegate to one of the contained models.
	getParticleTexture() - Dummy out with new ResourceLocation("missingno") (magenta + black checkerboard) or delegate to one of the contained models.
	handlePerspective() - Real meat here
		Depending on the TransformType, choose the IBakedModel to return
		If that model is also an IPerspectiveAwareModel (instanceof), use handlePerspective and store the returned Pair
		Otherwise:
			Use getItemCameraTransforms().getTransforms() to get ItemTransformVec3f
			Convert that to TRSRTransformation with the TRSRT constructor
			Use TRSRT.blockCenterToCorner on it
			Then convert THAT into a Matrix4f with getMatrix
			Pair the model and matrix with Pair.of
		From the stored ImmutableMap<TransformType, TRSRTransformation>, get the relevant transform and get its matrix
		Multiply the matrix in the pair with the matrix from the map
		Return a pair of the model and the new matrix

3) Create custom ICustomModelLoader class
	Super simple, just a dummy.
	accepts() - Pick one special ResourceLocation to be the entry point to all this that your item will use as its model, and check for equality.
	loadModel() - Also a dummy, just instantiate the custom IModel and return it.