Add '2+ interoperable implementations' line.

Co-authored-by: Sarven Capadisli <[email protected]>

social-web-maintenance-wg-charter.html

@@ -271,7 +271,8 @@ <h2>Success Criteria</h2>
       In order to update the Recommendation, each substantive change is expected to have
       <a href="https://www.w3.org/Consortium/Process/#implementation-experience">at least two independent
         implementations</a> of every feature defined in the specification, where interoperability can be
-      verified by passing open test suites.
+      verified by passing open test suites, and two or more implementations interoperating with each other.
+
     </p>
 
     <p>