Are you looking to overlap ECOG sensor locations, rather that signals?
For overlapping sensors, a GUI option is not available yet. However, it can be done, by exporting the sensors locations (in MNI space) for each subject, then concatenating those locations in a new channel file, and use that concatenate channel file for the default anatomy. Check this post:
For overlapping signals, can you provide an example of the expected result?
P.S. Make a new topic for a new question, this will make easier to locate related topics, and will help other users to find relevant topics for their own questions