You make them look at a movie, like
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by example. Or less explicit (and less on drama) it doesn't matter much as long as the movie picture a girl who struggle to decide we she love between two characters, or openly love two characters.
Then you make them talk about the movie, and slowly discover that they both are interested by the idea. To be more realistic, stick to this interest "for the idea", so purely intellectual, and way few days before a "you know, I was thinking about this movie..."/"Yes me too..." scene where they decide to give it a try. And after it happened once, make them talk about it once again, to share their feeling about it.
Or you can also look at
Icarus Media 's
My Girlfriend & My Futa Grandma, as example of how it can be handled when partly coming from a third person, and how love slowly erase the doubt and struggle.