Responses
I meannnn there is "Solar Eclipse" which matches the description somewhat- but the blonde hair and black hair dudes aren't brothers. The black hair dude hires uke who looks like the black hair dude's brother to pretend to be his brother (who is missing) and i guess in a sense spy on the crown prince (blonde hair dude).
Ya me and my hazy memory
The seme were two brother seme was blonde or may be white and the brother had black hair the black hair brother sent the uke to spy on the seme but the seme knew that the uke was a spy the seme even make the uke were a black gown oh and the uke were twin the seme help the other twin to hide .. . . I know it's all jumble but I remembered the plat can't remember the name