Quick tutorial:
You can increase your chances for that very simple. I created a whole yuri island with 6 girls (8 planned). Every girl has perversion and socialbility 50% to 100% higher than the other EXP. Then you need the H skills Lesbian Lover, Lust Devil and Heat.
This way you get a lesbian sex scene every 5 to 15 minutes (only the scissor and fingering scenes for now).
Just watch the map and when you see two girls standing in front of each other for about 30 seconds (talking to another) you have a high chance that they run off together to the next outdoor bed and engange (or special place, which isn't shown on the map).
If they part ways after talking, you were out of luck. This is the random part of the game.
EDIT: also they even engange with Shan