In the WikimediaMessages MediaWiki extension, the strings
should be merged into one string. You will also have to check in the codebase where these two messages are used and update the code accordingly. The new string also needs proper PLURAL support to be added.
You are expected to provide a patch in Wikimedia Gerrit. See https://www.mediawiki.org/wiki/Gerrit/Tutorial for how to set up Git and Gerrit.