There isn't a way to go back but depending on your choices so far you should still have the options for sharing Alice or Samantha
Stealing Alice should have given 2 chances to decide which way to go. I think the first time you can choose to steal is at the Church, and the 2nd final time to confirm steal or share is at the club after dancing with her a few times and she offers to meet you two in the toilets. If you're past this point, you're out of luck.
Samantha you need to share Ken with Samantha and run around the beach and mall randomly til Adrian runs into Samantha, but if you didn't share Samantha with Ken you might be out of luck