I would like to make a CW booking using a 2for1 voucher but am unsure how to book it to ensure I get the flights I want.

For example I want to book outbound LHR-GIG-EZE and return SCL-LHR
I want to take a 4/5 day stop over in GIG before continuing to EZE.
Something like
Day 1 LHR-GIG
Day 6 GIG-EZE
Day 17 SCL-LHR

In order to achieve this I will need to book over the phone t-355 but wanted to know if it is possible and how to achieve it.

- Am I right that in thinking I book a one way ticket for 2 using the 2for1 at t-355 LHR-GIG?
- Then call to extend it 6 days later to EZE (as that flight was not bookable and paying there slight increase in Avios)?
- Then call to add the return SCL-LHR 17 days later?

Or do I wait until t-355 from the day I want to travel to EZE and book it but ask them to make the first outbound leg 5 days earlier - I assume that this probably would not work as all the reward seats will have gone on the LHR-GIG leg by then.

Or is there a better approach?
Thanks for any advice.