The twins got out of their apartment after making up their minds to go talk to Dongwoo.
They played a lot of scenarios in their head that could unfold after ringing his bell. Something like Dongwoo checking the intercom and then pretending not to know they were at the door.
Or maybe he would answer and make an excuse that he was already on his way to bed and would see them the next day.
And if he did open the door, he just might tell them off and say they were just being paranoid and that the reason they hadn't spent time together was because they were all busy.
And if they tried to bring up his previous statement about him being selfish, he might conclude it to be a slip of the tongue or that he did not remember.
He could do so much, just to avoid speaking with them and they were prepared to try everything just to get him to talk.