I found one in the past, I'm 90% certain. However, after hours of searching, I haven't relocated the web page that had this splitter/Y cable. If you know of such a cable, please let me know! Thanks.

 

An RS-232 modem splitter is available from Black Box. While not a cable, it is a relatively small box with a 120 VAC input and multiple ports. I have not used it with Modbus, however.