To search the content of more than one site at time, DuckDuckGo provides the site operator which can contain more than one domain. To search both Macdrifter and Hobosigns I created this Workflow for iOS.
Prepending a search phrase with "site:macdrifter.com OR site:hobosigns.macdrifter.com" does the trick with a DDG search. Sites can even be across different TLDs. So, if you want to search 5 or 6 sites at once, consider a canned DDG cross site search.