You can load a specific config file like this :
Configuration config;
ExeConfigurationFileMap ecfm = new ExeConfigurationFileMap();
ecfm.ExeConfigFilename = <your_config_file_path>;
config = ConfigurationManager.OpenMappedExeConfiguration(ecfm, ConfigurationUserLevel.None);
var mySection = (BusinessConfigurationSection)config.GetSection("GroupName/SectionName");