I get the feeling that Riko is aware that You's amnesia is mostly feigned — but how aware You is of Riko's knowledge is iffy. There's obviously something that is preventing them from admitting things, and it doesn't feel like just cowardice. Is one, or are both of them, cheating? Y'know, just to compound things.
Or it could just be that the first time, You genuinely forgot, or impulsively lied about forgetting, and Riko is genuinely unaware and keeping the pattern up out of warring lust and guilt, and You ditto. It at least doesn't feel like this is the first time You has remembered. Hrm. This is just the sort of story that keeps my mind poking at it.