For presentation purposes, it would be nice to be able to have groups of headings. In this case, the headings would be in two rows - the first with the group names that span over the headings in the group, and the second with the actual headings.
In terms of how to set this up, maybe just some separator in the column name would create a grouping. e.g. “Email Invite V1/Clicks” and “Email Invite V2/Signups” would be grouped into “Email Invite V2” with subheadings “Clicks” and “Signups”.