Overwrite conflicting resources behavior

Hi,
As I understand, when client 1 is logged in using JID, and client 2 tries to log in with the same JID (exactly the same, including the resource), client 1 gets disconnected. Is there a way to overwrite that behavior, so that client 2 won't be able to login if that JID is already in use (e.g. opposite of what it is right now)? Any hints on where (source files) this behavior is implemented? Thank you