This might be an impedance transformer. I haven't EM simulated this structure, but I'd strongly guess that a CPW line with a split center conductor has a different impedance than a similar line with a "normal conductor".
The reason for using this structure would probably be that the CPW ground plane will stay untouched, and the location and impedance of the "different" tuning line can be easily changed, even with a knife in prototyping phase.
In a normal microstrip line you can add tuning stubs of different width and length quite easily as the ground plane is not on the way. You can (and sometimes even have to!) do this with a small piece of foil and a soldering iron when you want to tune the circuit to a good performance. With a CPW line this method is generally not possible or at least easy.
Hobbie