I am soon to finish a DONE4, and the last segment is
HKG-NRT. I will be starting a new DONE4, and from
Japan I will go directly to Europe (probably KIX-HEL).

Since it is cheaper to buy my new DONE4 starting
in HKG rather than Japan, this will leave me with two
HKG-NRT tickets.

I assume if I don't fly the last segement of my first DONE4,
nothing bad will happen. However, is it possible to use
both tickets on the same flight to give me an empty seat
next to me and double FF miles?