Sounds like OP has confused things. A CX account would allow you to collect miles in CX, which are not the same as Avios in BAEC. Obviously since both are in oneworld the set of flights you can credit to each is going to be mostly overlapping, but there are not a huge number of reasons to have two programmes in one alliance. What are you trying to accomplish...? If you intend to credit to BAEC you have no need of a CX account.