From Kurt's pool tutorial:
Code:
ch_Kurt "Also, if two of zee frauleins don't seem to like each other very much. . ."
ch_Kurt ". . . bringing zem to hang out at zee pool with you is a good way to help better zeir relationship viz each other."
That's basically all the official info about making them friends afair.
If you got more questions about game mechanics, just ask and I'll dig it up from the code.
Later in chapter 2 when you basically start over with another three girls, this could get interesting if you are already in poly relationships with the chapter 1 girls.
I guess the same careful approach is necessary then to rather increase all their stats first before making them your girlfriends as well.
But first let’s see if and how many quests the chapter 1 girls still get then, so you can easily avoid flirting in public and going to dates with the old girls to avoid cheating in front of the newer ones.