To me they would be much better off to end ALL THE SERIES WITH THEIR OWN WINTER FINALES BY the First week of December....with each last scene a big cliff hanger leading into the crossover...
The crossover would then be a solid night for CW the second week of January have it a 2 hour movie extravaganza (just make sure that you are not on the same night as the National Championship game....and they all start their 2nd half of the season the next week.
So, they all have 7 episodes filmed for the first half, the 7th being the Winter Finales for all the shows....or 8th which ever works out with scheduling, I WOULD SAY 7, and the crossover would be filmed during the time they filmed the 8th episode this season....then they finish their shooting schedule with 9 and possibly 10 take their break as usual.
I totally have no clue what I'm talking about, but that would be a good scenario if they could schedule it.