It's currently not possible to link a Phabricator account to more than one external account of a given type (where "type" is one of "Facebook", "Google", "GitHub", etc.). In hindsight this was an architectural mistake, but I didn't think about it at the time and left us with a mess to clean up. It will be resolved by {T1536}, which is a sort of umbrella task for remedying various missteps on the auth pathway. We've made some progress on that, but it will be at least a little while before it lands.