Yes i'd keep them in as long as possible, its so cold out anyway they'd probably be happier in-door for now!
But i think its good to encourage the cats to sit on the window ledge & look out at their surroundings so they can become familiar with all that is around them! Also when you do let them out at first make sure they are hungry, then hopefully they won't want to say out too long coz they'll want their dinner! Keep an eye on them & maybe put them on a lead at first.
I look after a colony of feral cats, they live in kennels in my garden & in my shed. They are completely wild so they would never settle to in-door life. All of my feral cats are neutered except for one, i've been trying to catch her for months but she is a clever girl, i will get her eventually! Anyway she had one kitten recently, the kitten was born in October so the weather had already turned cold, so i was very worried about him.
So i have taken him in & tamed him, he is gonna live in-doors with my family & i. I also have a dog & thankfully my dog & the kitten are at last getting along really well! But i do eventually want to be able to let the kitten out to see his family in the garden once he has had all his shots. So i'm always sitting him up on the windowsill so he can see them & once a day i carry him out to say hello. I want to make sure the other cats stay failure with the kitten so that they accept him when i do eventually let him out.
But i'm hoping that sitting him on the windowsill & carrying him out to the garden will help him to recognize his home.
I think your two cats will be fine! Cats are so clever & they always seem to remember where home is! Its also probably a good idea to put a safety collar on the cats with your phone number on it in-case they do wonder off! Best of luck & make sure to let us now how they get on!
