Far too many things would have to align for something like that to happen.  Just getting Spider-Man in the MCU required a buttload of extra work, and that's ONE character.

MCU and DCCU combining?  WAY too many chefs in the kitchen, too many different people to appease, and since the profits would be split down the middle?  Too much work, not enough reward.  MUCH simpler movies that both parties WANT to get made don't get past this stage.  A friend of mine was on a TV show that was quite popular and got cancelled because the rightsholders started disagreeing on sh*t.