Flyerbunny

We have an Amex 2 for 1 and wish to book an award flight, CW or F, outbound and return flight dates approx 5 weeks apart.
How should I do this?
I read somewhere about booking the outbound only online with the 2 for 1when seats are released and then phoning to add the inbound later when seats are released.
Will the 2 for 1 definitely still apply to the inbound if I do it this way?
Cost of this?

Is this the best way to proceed?
I would prefer not to have to phone to add the in bound if there is another way to book the whole trip online as award seats tend now to be snapped up before the phone lines open.

I have no status.

Advice gratefully received!