Use model.component(<ctag>).geom(<tag>).feature(<ftag>).selection("input") to select the faces to concatenate. The default selection is empty.
Use ignorevtx to specify if the feature also removes the ignorable vertices on the boundary of each resulting composite face.
Use keepformesh to keep the input faces while meshing, to help you in constructing the mesh.
A COMSOL Multiphysics standard cone has six faces. Using the following composite face operation, the result is a cone with three faces: top, bottom, and side.