Combining segmentations in ITK-SNAP to create a brain mask

Hi all, I’m attempting to follow the protocol in Appendix A of this article to create a brain mask for QSM by combining 2 segmentations (from the 1st echo and the last echo). However, it doesn’t go into the specifics of how to combine 2 segmentations from 2 different images. Any thoughts on how to do this in ITK-SNAP? Any advice would be appreciated.