You can have multiple server profiles and any number may be connected at any given time. Only the server for the current primary file is active. When you switch primary files, the server of the new primary file becomes the active server. When you explicitly disconnect from a server, all tables that were open on the disconnected server are closed.
All commands that execute on a server are executed on the active server. A user cannot specify a different server for each individual command.
Note: Your User ID and password are only validated each time a new connection is established.