Koibito no Namae

Story 1: Lover's Name
From Ultimate Manga Guide:

One night, Ei Ariga runs into a very obnoxious playboy stranger. Then he finds out that the guy looks exactly like but otherwise is totally opposite to the timid and very polite Ryouichirou-san who lives alone in a very big apt and has hired his services (Ei-kun's father is a private detective), to find out why he doesn't remember doing some things and being at certain places. Ei-kun discovers that the nasty, violent and annoying carbon copy is called Naruaki and things get extremely complicated when he starts falling in love with both Ryouichirou and Naruaki...
Lover's Name was re-printed in Double Lover (2005) with oneshot Kimochi no Yukue.

Story 2: Saikou no Kimi was re-printed in Saikou no Kimi (2007) with two other short stories.