I can't figure out how to post screenshots on this, but the second panel on page 21 of the first chapter has Hakari and Karane introducing themselves, and they mention them being in the same class, neither of whom were surprised at that. other than that there's no conclusive evidence one way or another, as they don't explicitly state whether they do or don't know, but in my opinion it looks like they at least knew each other.
even if they didn't know each other, as you said, they never fell in love upon meeting each other for the first time. however, the first zing was a three-way zing between Hakari, Rentarou, and Karane, who's to say all three didn't zing, and not just Hakari/Rentarou and Akane/Rentarou. there's no real evidence for that, so I just prefer to believe it, but even if that isn't the case and they aren't soulmates, they've clearly got something going on, and one of them is a major tsundere so it's not like they'd immediately confess to one another. and before you say "but Karane confessed to Rentarou, then why not to Hakari?" she only did it because Hakari did first and didn't want him stolen by her, if she was the solo heroine we'd have a whole 5+ arc manga before they even kiss
