I agree with the posters so far - my son was 18 months old when my 2nd son was born, and we not only coped but enjoyed the experience! Being organised is key; I had a basket with wipes/nappies (both sizes!) etc, so when one or the other needed changing I could do it in the lounge and not have to leave toddler or baby.
My son didn't walk until 15 months, so for a lot of the pregnancy I was carrying him around, which was probably the most difficult thing (I suffered with back and pelvic probs). As your little one sounds a bit older, it might help you if you can encourage him to be as physically independant between now and then (simple things like him climbing into his car seat himself, which might make it easier when you're heavily pg).
Don't know how you're eldest is in pushchairs, but I'd have been lost without my 360 nipper twin, I lost most of my baby weight pushing the 2 of them around the town or the local park. It was suitable for the newborn and the toddler, and as they got older they could chat and play side by side. My eldest is now 4 and I'm gutted he'll no longer sit in it as it made life much easier!
I know it sounds trite, but it will be wonderful, the best thing we ever did for my son was give him his younger brother, they're really close now and company for each other. When I was pg first time around, experienced moms just loved telling me I didn't know what I was in for, and then when people learned I was pg so soon again they just loved telling me how I was going to struggle with 2. In my opinion lots of people love scaremongering! And I'm now 15wks pg with number 3 so it can't have been that bad...!
Try not to worry, make plans but don't drive yourself mad with things which might never happen x