Based on my experiment it doesn’t seem to work at the moment, so this is likely a feature request.
When working on various complex objects (e.g. faces) it sometimes is necessary to create several external mattes for the same shot. MP’s magnetic and brush spline tools are fantastic for fast work.
However, at present each matte has to be exported into a separate sequence. Other tools can take advantage of EXR multi-layer and embed each matte as a part into a single sequence which simplifies file management and importing on the other end.
In Silhouette there is a Multipart Output node that accomplishes that.
MP is almost there. I can select EXR and I can select ‘all’ or ‘all selected’ mattes. Except it appears to merge the mattes and write them as one part. There should be an option to write ‘all’ or ‘all selected’ but then write them as individual parts into the EXR sequence.
I can do the roto in MP, export Silhouette shapes, bring them in Silhouette and render multi-layer there… Just an extra step at the moment.