Brood pattern? Queen or laying workers?
You need to be very careful, if uniting. Shaking out is one quick option. Another would be to place this hive adjacent to your good hive for a week or so and then move it entirely away (say, 5 metres) on a warm flying day. That would allow flying bees to transfer. Then shake out the rest.
Shaking bees out on a cold day might not be good news for them - particularly if mouse guards are still in situ or a restricted entry. Bees don't swarm, at this time of the year, if there is little brood and loads of laying space. Please just think, before making inappropriate claims. Yes, bees do swarm in March - but not likely this season in the UK, I would suggest!