I think you need an angle if you're going to flirt with other girls when on a date. Like, "oh, She's a friend of a friend", or " She works at Starbucks and I'm really curious how their operations work" or any other lame excuse/shameful lie. She won't be offended if you've conveyed that message in a calm and non-chalant manner, after all, whats there to be offended about. You two are not yet exclusive and she has no claims over you. She can't expect you to give her all your attention coz your not an AFC.
What you'll incite in her is JEALOUSY and this can be such a powerful tool in increasing IL. Imagine back when you were jealous about your target flirting with someone. Did it put your mind into overdrive thinking of ways you could make her yours. You probably already made her yours in your mind and feel anyone else talking to her as a threat on your property. The same applies to your date. Not only will she see you as of higher value, but your advances will be met with less resistance, because her natural instinct is to deal with competition and she doesn't want to lose.