Historically, taxi radio systems have used two frequencies without a repeater. I believe the idea was to prevent drivers from jumping on other calls or something.
In the US, there are defined pairs for taxis -- well, there used to be at least -- so you could look up the other half. DOT -> DOC -> IC has always avoided doing things the way the FCC does, so it's much more of a crapshoot.
Maybe, using the TAFL data, you could figure out the name of the company you are listening to (since you know one of their frequencies) and then searching for other frequencies assigned to the same company?