Depends on how weak. And if it is weak for a reason, like disease. Asses that first. Then, I would rather take the weak healthy hive and place it on a stronger hive. Then the two hives could balance out, and the weak hive could get heat from the stronger one.
Here is what i do
Force the strong one into one box. Spritz them with vanilla or mint flavored syrup (cheap spray bottle) I spritz the top of the box and the bottom, make sure you can smell the syrup. This masks the queen's scent. I do the same thing with the weak hive. Then the strong hive gets one sheet of newspaper on top with a small slit or two and a queen excluder. Place the weak hive on top. In two to three weeks (prefer 3 weeks) split the hives and add another brood chamber on each.
Now one of two things will happen:
1. you will see two strong hives ready to split and add a brood chamber on each right away since the queen will be close to running our of room
2. The strong hive will kill the weak hive's queen and the bees will combine to make one hive. This should not be looked at as a failure. Maybe the hive was weak for a reason.