The words "and the port used" in the second paragraph of this section of RFC-959 are erroneous (historical), and they should be ignored.
On a multihomed server host, the default data transfer port (L-1) MUST be associated with the same local IP address as the corresponding control connection to port L.
A user-FTP MUST NOT send any Telnet controls other than SYNCH and IP on an FTP control connection. In particular, it MUST NOT attempt to negotiate Telnet options on the control connection. However, a server-FTP MUST be capable of accepting and refusing Telnet negotiations (i.e., sending DONT/WONT).
Although the RFC says: "Server- and User- processes should follow the conventions for the Telnet protocol...[on the control connection]", it is not the intent that Telnet option negotiation is to be employed.