Couples I've known who both had double-barrelled surnames before marriage generally combined one of the names from each of their names to create a new double-barrelled name, which is what their new family and kids use. E.g. A-B marries X-Y and they become B-Y or X-A or whatever.

Sometimes, the missing parts of the surname get given to specific children as middle names. Which is a nice way of acknowledging older family members without burdening your kid with an old or boring name.