I'm kinda surprised actually, with how they were going, that they actually did talk so relatively directly about romance. I expected them to fully bury it until long after they're adults, so that implicit promise of "look we'll date later after we've made sure we don't cause any parodoxes with this flashback" has interesting implications for their present day 'friendship'. Maybe they've just been on the dl for as long as the kid has know them both lol

Also the relatively casual way it was brought up, with neither reacting like their shared romantic interest is news, possibly implies they had an offscreen fling beforehand.

I'm fairly certain they haven't. I think after the rejection in the flashback, Youji tried to let her feelings go, but Youzi's pushing is influencing her. I think Youji has been holding her feelings back for fear of being rejected again, and Weiwei has been holding back because she doesn't know if Youji is still into her that way.

When Youzi first suggests Ren Wei as a love interest, Youji says something like "Don't invest real feelings in your ships", I think that's because she's trying to supress her own feelings at this point. But Youzi's pushing is having an effect on both of them.