Well, according to (very old, so don't shoot me if I'm wrong) tumblr asks, you can flirt with ro's as long as you don't get locked into a 'route' beforehand- so, let's say I have a childhood crush on Nimue, but then flirt with Galahad, BUT I also sometimes flirt with Nimue as well. In game, it would probably translate to the two characters experiencing confusion regarding who I'm interested in, and potentially start a few arguments. (At least, from what I understood from, I'll say this again, very old tumblr asks.)