To add a second date period to an external link, it currently requires to add the link a second time. It isn't intuitive given that relationships are grouped by URL.

An improvement would be to show the button "Add a new relationship" when at least one relationship for that URL has a date period (begin date, or end date, or ended flag).

Example of legitimate use case: Artist "Example" has official homepage at http://example.com from 2010 to 2011 and from 2016 to present (because the homepage was down or elsewhere in the meantime).