The only girls you can share with him (afaik) are Alice, Samantha and Anna, if you don't want to share anyone with him then you can't go to Japan since most of the scenes are with him, if you're choosing the Anna-Adrian sharing route then you get the injector (Idk if it's written like that) and get the Hacker perk from Sam to hack his PC on thursdays, after that you tell Anna that he's subscribed and should seduce him, after a while he'll tell you that you should go to Japan together sometime since he's thankful.
Personally I'd recommend just going with Sam since you (probably) are already sharing Ken with her and can avoid the scenes if you don't invite him and don't visit Sam on fridays.