ProSoft makes a DF1 gateway that functions as a multiplexor. This permits up to 3 DF1 master devices to communicate to a single SLC or MicroLogix processor while only using a single communication port on the processor. Additional information can be found on the products web page:
http://www.prosoft-technology.com/Products/Gateways/DF1-Serial-Port-Expander-Gateway/DF1-Serial-Port-Expander-Gateway