i have a similar question:
i have my flight PIT-LHR-HKG-LHR-PIT. the flight between PIT and LHR is on UA, flight between LHR and HKG is on aeroflot, and they are on different tickets (in order to get low fare). problem is that between LHR and HKG i have only 20kg max.
is there a way to increase the luggage allowance? thanks.