Powerline modem ST7580 single channel mode
I know this question is old, but yes it is possible to use one carrier at a time.
This is explained in the UM0932 User manual throughout the document.

You can set up your modulation frequency upper and lower, on page 36/34 of the document you set a bit to choose the upper or lower frequency. This is the MIB setup section of the document. Important to read through all of it as the default setting will not work for your altered schematic.

You can also select the frequency you wish to modulate at in the DataRequest CDM on page 25 and 26.