How to add a booking on the CX website
 

I have one booking entered my LAX-HKG (booked with Alaska miles) and I would like to add the return booked with BA Avios. I can't see how to do that if it is even possible. Thanks

I don't know exactly what you mean by "add" a booking, but you can see the booking on CX's website, and change seats and so forth, if you know the CX record locator, which BA can give you. It is also possible that BA's record locator works on CX's website, since I seem to recall that BA uses Amadeus.

My problem is that since I added the Alaska one first I don't have the option to add a second. Maybe I'm missing something.

Still don't understand what you are trying to "add." You have a PNR booked through AS and another book through BA. They are 2 separate bookings and you can't possibly combine them, if that's what you are trying to do.

After using Manage Booking with the first PNR, you have to "log out" (even though you haven't actually logged in) before you can enter the second PNR in Manage Booking.

There is no addition or subtraction involved anywhere.

You cannot add another CX booking (booked with AVIOS) to another CX booking. They will show up as two PNRs.

With your AVIOS booking, as BA uses AMADEUS, you can easily retrieve and view your booking on CX website.