E.M.
My son was 22 months old when our daughter was born and he was not in the room because I ended up having a c-section, but in hind-sight I think that was good because I think looking back on the whole situation he probably would have not "gotten" at all what was going on and would have been kind of freaked out.
I'm an attachment mommy too - I think the BEST thing I've done to help with the transition from singleton to sibling was to carry the new baby in the sling every moment of the day when she wasn't nursing, so I could have my hands for my toddler. With the baby tucked into the sling, he would almost forget she was there and that actually helped I think. Also, this helped the baby to be happier and not cry hardly at all, which was good for me too.
Also, and this will probably be unpopular...but I HIGHLY recommend that you begin co-sleeping with your older child so that you can recharge her battery at night. You may already be cosleeping since you are an attachment parent, but if not, I can attest that this practice really helps the older child to get the snuggles they need - and that you need too. It actually works really well to have both kids in the bed - baby on the outside next to you and either the wall or a bedrail, older sib. in the middle of you and hubby - or just over on the outside next to hubby with a bedrail.
Enjoy every minute of this time - your life will never be the same when baby #2 gets here, but it only gets better. After a week or two you'll wonder how you ever lived without and were happy without your new baby!
Good luck, and just know during the hard times that it does get easier as they get older.