You can connect to only one external system at a time from the Salesforce set up UIs.
However, a provider may want to connect to multiple FHIR servers as the data may be distributed across different servers.
Workaround: The FHIR server credentials can be passed in the HTTP header parameters for each individual run. These would override the credentials entered while configuring the Generic FHIR client through the Salesforce set up UI
Note : Current limitation is that access tokens would be generated each time the parameters are sent in the header