Need Help. Spliting household and taking some selected furniture.

Move with them and after remove them from household. Or evict them both out right now and move with all the objects.
If you want the ex to have their old place just free real estate to move them back in

So here's what I think I would do:

Step 1: move anything you want to take to his new house into the household inventory.
Step 2: leave anything you want his ex to keep in the current apartment.
Step 3: Evict them together.
Step 4: Move them together into the new house.
Step 5: place the items you want him to keep in his new house.
Step 6: Move the ex out of the household and back into the old apartment, which you will purchase furnished so that all her old furniture is there.

No mods required; at most, you might have to do a money cheat to make sure both of the new households have enough cash for their respective homes.

Thanks. Using this advice I got them successfully moved. :)