Don't do that.

Presumably this is outside North America, right, since in NA round trip=2 one way?

Simply buy the round trip CCC-BBB-CCC. Throw away the return. If as I suspect this is a codeshare, even better to buy it with the operating airline.

In case they would later charge your CC for the difference (which in some European countries such as the UK they might try) contest the unauthorized charge.