I doubt you will see two Flash's in the movie. DC has messed up the Flash's existence on the CW the least they could do is take their time and get Flash right in the DC universe. Starting with they should have made Barry a more mature face, so they could intro Wally in the JL saga. But going forward what they should do is connect CW Flash to the DC universe. Barry and Iris should have a mixed race son name Bart. Barry should help the League in the parts one and two and decline to join the league in favor of taking care of his wife. Remember Barry lost his mother and he doesn't want to lose Iris so he will remain behind, although Wally is gung-ho about the opportunity and he loves being around John who is like a big brother to him, he is not sure about Hal, but Barry co-signs so he is ok, Since Barry walks away, Hal declines as well, At some point John convince Hal to join. Barry discovers the Speed force and gives the mainline connection to Wally West because Barry wants to protect Wally any way he can.
These moves lead into the Green Lantern movie with Hal convincing John to embrace his full power and join the Corps.
Also, this leads into Hal and Barry being Friends, and if the Flash and Green Lantern movie is about Barry and Hal here you go.
Yea, you could have a small part in the Flash and GL movie for Wally and/or John. But connect the bridge just takes a little thinking however, it is not hard because all of these guys the entire league thanks to Batman always have someone watching their backs, if not the whole team.
I mean there has to be a big Doomsday battle brewing or even a Darkseid battle brewing where Superman is getting trash and for once the team comes to save his bacon...
Once you get the founding group off the ground you can go into the unlimited groove.
You could even venture into Hal Jordan sacrifice to Parallax arc. You could lose Hal in a JL film and retrieve him in not the 2020 GL film. It looks as if the 2020 is going to be a retro on John Stewart and maybe some Kyle Rayner. But in maybe a 2024 movie...