How To Keep An Orphaned Kitten Comfortable?

I have 2 kittens , 1 is about 4 weeks old, the other one is 8 months. I want to keep them separate for now, but not forever. Can anyone tell me how long it would take to get over this separation anxiety? The older kitten seems very scared of anything new and will hide under my bed or behind things all day if he’s home alone without his sister.. even when she comes in with him..

Just wondering what you guys do to help kitties recover from being separated too much… maybe give them time out for awhile away from each other, etc… any advice welcomed!!