I wrote some software for doing this a while back and you can grab it on matlab's community page: https://www.mathworks.com/matlabcentral/fileexchange/59161-volumetric-3
It's capable of overlaying and rendering multiple sources of volumetric data so long as they are in the same voxel coordinates and stored as 3D arrays. It also comes with a full GUI for editing and creating volumetric layers. It uses a similar technique to Vol3d, but its got a much more developed interface and GUI.
Disclaimer: I wrote volumetric 3, but it's essentially shareware so use it if it works for you!