It's because the uke was meeting the supposed twin at a restaurant. The two of them said goodbye, and the twin left. When the uke got home, the other twin was already there, cooking and wearing different clothes. It's strange that he got there before the uke even though they left at the same time.
There's also that part in one of the chapters where they're kissing in the living room, and he says, "I should be the one hearing that today."
I also think they moved closer to the uke so it would be easier to switch clothes and locations. There's another thing I noticed in Chapter 18! The seme had already seen the uke wearing new clothes. He goes to buy coffee and then shows a surprised expression when he sees the uke in different clothes. I assume they changed clothes in the café bathroom

YOOO i just noticed that the mc is holding two collars, which is red and blue, the blue is wrapped to the dimple one and the red deems to bee.. Not shown? Yet, so there really is a twin y'all