SELECT sum(market_rate) FROM furniture ORDER BY market_rate DESC LIMIT 2
find the total market rate of the furnitures that have the top 2 market shares.
[Schema (values) (types)]: | manufacturer |  manufacturer : manufacturer_id (text) , open_year (number) , name (number) , num_of_factories (text) , num_of_shops (number) | furniture : furniture_id (text) , name (number) , num_of_component (number) , market_rate (text) | furniture_manufacte : manufacturer_id (text) , furniture_id (number) , price_in_dollar (number);