I understand how scared and worried you are - I'm also pg and have a young baby, although I'll have around a 14 mth gap between mine which will make it a little easier. I'm 13wks pg and have an 8mth old. I think we'd be crazy if we weren't worried about how things will be but I'm just really trying to stay positive and remind myself that although it will be hard, I'll get through it and it'll just keep getting easier as time goes by.
Everything seems so much worse when you're so sleep deprived and you know that soon things are going to get even harder, too. Luckily Adara's sleeping really well again for me but from 2wks after conception until I was 11wks pg, she was waking up hungry 3-6 times every night, as she was breastfed but I had lost my milk so she was losing weight. We recently found a formula that she would drink and now she's much better. Have you considered taking him to a sleep school? It worked for a friend of mine.
I agree that if you do have family or friends that you trust to look after either baby (or both, one at a time so you can spend time with the other one) you really should use that. Just try and get as much help as you can to get through it. Is your partner able to take some time off work when you come home from hospital?
What have you organised for the labour and birth? I'm getting my MIL to bring her to the hospital when it's nearly over so she can come into the room as soon as I've delivered, and my birthing centre will let her and my husband stay with us in our room for as long as we stay there. I don't think it'd be a good idea for such a little one to see me in so much pain but I do want to have her close to me afterwards.
My biggest concern isn't about the lack of sleep and coping with the babies and housework, etc., it's whether I'll be able to spend as much quality time as I'd like to with each one and how Adara will feel about sharing me when she's so young.
I'm sorry, I can't really give you much advice that you wouldn't have already worked out yourself as I haven't been through it yet... But I'd love to chat if you'd like to, my msn is firstname.lastname@example.org. Let me know how you go with it, even if you don't want to chat (or have time to!).
Wow, sorry it's such a long post!!!!! Good luck with everything!