Hi Ricard,
yes FasterCap supports this case. You need to split the surface panels into sets, where each set is composed by all panels in contact with the same dielectric. You then specify them in different files and in the main file you reference them via 'C' statements with the proper permittivity specified and join them using the trailing '+'.
You can find a full example under the name 'capacitor.lst' in the Samples directory that is installed with the FastFieldSolvers bundle.
For your reference I copy&paste the main file (i.e. the 'capacitor.lst' but not the other files) here. I highlighted in the following the permittivity of the different set of panels belonging to the same conductor:
* Capacitor
*
* This capacitor is made by a dielectric material with relative permittivity
* equal to 3.0, sandwiched between two square metal contacts
* with finite thickness
* lower contact
C capacitor_contact_sides.txt 1.0 0.0 0.0 0.0 + 0xd8d8d8ff
C plate.txt 1.0 0.0 0.0 0.0 + 0xd8d8d8ff
C plate.txt 3.0 0.0 0.0 0.2 0xd8d8d8ff
* dielectric medium
D capacitor_diel_sides.txt 1.0 3.0 0.0 0.0 0.2 0.5 0.5 0.5 - 0xa793b4ff
* upper contact
C capacitor_contact_sides.txt 1.0 0.0 0.0 0.8 + 0xd8d8d8ff
C plate.txt 3.0 0.0 0.0 0.8 + 0xd8d8d8ff
C plate.txt 1.0 0.0 0.0 1.0 0xd8d8d8ff
Best,
Enrico