This is not a silly question at all. Unfortunately, you cannot do "mixed class bookings" through CX's website. Not only that, you also cannot do "mixed sub-class bookings" (e.g. E class for the outbound sector and R class for the inbound sector) of your choice through CX's website - the system will simply offer you the lowest available sub-class. If you wish to upsell the sub-class, both the outbound sector and inbound sector will end up with the same sub-class.

The only solution is to call CX's ticketing office or ask a travel agent.