On January 4th 1923 the first simulcast occurred. A concert live on 660 WEAF-AM in New York City was carried on 1230 WNAC-AM in Boston. the connection between the two stations was not by STL, it was by telephone line, leased from Bell telephone.
The word "simulcast" did not yet exist. They called it a "chain broadcast." If you're wondering how WEAF explained this all to the phone company dont. AT&T owned WEAF making the experiment much more feasible. At 8:03 PM Davera Nadwernay sang the song "Habanera" from the opera Carmen by Bizet. That broadcast only lasted 5 minutes, emanating directly from Carnagie Hall. (While this is the ersion I beleive, other sources refer to this as a solo saxophone performance)
For KDKA thsi was less than a year after they received the first commercial license from the FRC. This was also of course the very beginning of network broadcasting. Only a few months later they attempted a bigger simulcast involving 810 WGY-AM in Schnectady, KDKA-AM in Pittsburgh, and 1500 KYW-AM in Chicago. This too was successful. AT&T enthused by the success of the two projects built a station in D.C. 980 WCAP-AM to advance their ability to syndicate programming. The facilitiy they built at WCAP for the purpose of linking staitons by phone was called the "Red" layout. This became the NBC Red Network, which formally launched Sept. 9th, 1926.