Welcome to the Forum.
No, it isn’t a good idea to clean out a hog box until/unless the hogs have completely vacated i.e. aren’t using it during the day either. You might find that they have somewhere else in mind for hibernation – they don’t tend to stay in the same nest as they have used during the ‘hog season’.
If there are two hogs, it’s possible that it’s a Mother with a slightly grown hoglet, but could also be 2 hoglets of different sizes. If one of them is a Mother she may go off and hibernate and leave the other one to potentially hibernate a bit later.
I think it is a good idea to leave out medium sized leaves as well. They are the ideal material for a hog to make a hibernaculum – partly because they are able to layer them and make the structure waterproof.
The recommended weight for hibernation is 450g. But if you are providing food and water, they are well and normally active and are attending for food every night – bearing in mind the mild weather – I would be inclined to leave them be if they only weighed 350g and keep an eye on them and keep offering them good quality food (cat/dog or meaty hog food). They put on weight incredibly quickly when they’re small and it would be less stressful for them than taking them in to be over-wintered by a hog carer. Hopefully they will both weigh more than 450g, so you won’t need to worry too much.
If you are going to weigh them, try to do that outside, so that it’s less stressful for them. Then if they are an ok weight you can release them again immediately.
Be aware that some hogs (especially hoglets) choose not to hibernate. If those there are amongst those who don’t hibernate, just keep feeding them and providing water all winter.
Good luck, I hope all goes well.