I'm trying to book a multi-city flight from usa to hong kong; hong kong to tpe, and then tpe to usa. I want for the long-haul portions in premium economy. Seats are available for the long haul flights in premium but no premium is available for the hong kong to tpe portion so the system won't show me any evening flights (which is the time I desire). Plenty of seats are available in econ for the tpe-hkg trip in evening.

How can i mix travel classes, take premium on the long haul and econ for the short haul?