Hi Xiang,
Using 3dVol2surf (or the vol2surf plugin), you can specify an
output dataset to read into suma. If you create and load 2 such
datasets (into suma), you can display their mixed colors at the
same time, with some amount of opacity.
ScaleToMap should no longer be needed, I believe. If you create
a pair of DSET.niml.dset files to load in suma, that should be
enough.
- rick