Cathay Pacific Extra-legroom seats question on two segments flights

I will fly Cathay Pacific for a long haul flight connecting in HK Sydney-HK-London. Looking at the booking options for seats, I noticed that extra legroom seats can be bought, but have an option to "get this seat for free if it's still available once check-in opens".

My question: when check in opens for my first segment (Sydney-HK), would I be able to select a seat for the second segment at the same time (HK-London)? ie selecting the seat before check in opens for people who only take the second flight?