Besides fan art the only way DC's superheroes will ever collide is through fan art unfortunately for now. However there are a few reasons why that can potentially be a good thing for both universes.
1. Two Actors for One character means more chances for us to see our favorite superheroes
I know that most fans were first upset when Ezra Miller was announced as the flash but why not have two flashes? Think about the possibilities of having the Flash from October through December then having a winter break and getting the Flash back in March. On top of that we also have the Movie Flash so during the summer break in 2016 we might even get a chance to see the Flash in Batman V Superman: Dawn of Justice. Who can complain with this many chances to see our favorite scarlet speedster?!
2. If Grant Gustin and Stephen Amell did get into the movies what would happen to their shows?
In the event that they did get cast how would that affect their shows? Would they even have the time to guest star while continuing our favorite shows? How would that affect our actors? Would the quality of both movies and shows dissipate due to lack of time? These things can be avoided by having two separate universes.
3. More development for our heroes
With separating the universes you have more chances to explore different story lines and develop new shows and characters through the tv shared universe. After Oliver Queen calls it quits we could have Roy Harper or Conner Hawke or Thea Queen take over the position of Green Arrow. We could develop shows like a stand alone The Question show which could feature The Huntress. We could develop shows like Doom Patrol or Metal Men. Or Even an injustice league minus Lex Luthor. There are so many possibilities for the DCTVU they could go on for years doing this.
On top of that
We could just have a crisis on infinite earths cross over eventually after Arrow and The Flash and all the other shows finish up their seasons or in between them and just have very small appearances.