My children are a week or so short of 18 months apart. My daughter's now over two, but I did have 6 months of them both being under 2!!
Two words: HARD WORK!
BUT, 1000000000000000000% worth it.
How they are with each other? They're actually really good with each other. They adore each other. I was terrified that Robyn would resent Logan, hate him for coming in and 'taking mummy' but I just made sure that I didn't say 'no' to her alot, and I paid them BOTH attention, at the same time and individually. They love playing together now
How much harder is it than having one? Yes it's harder, obviously. When one is settled, nine times out of ten, the other one won't be. You don't find yourself having much free time to yourself, and by the end of the day you are absolutely pooped!! But I wouldn't have it any other way. I may be exhausted, but I'm happy.
If you take them out, how do you do it? Now, in a double buggy. Or a single buggy and Robyn walks or uses a buggy board. When Logan was littler, I used a baby sling, and then I had an option of whether to make Robyn walk or use a buggy. Again, it's fairly hard in the beginning, but you soon get used to juggling them.
How did you find looking after LO while you were Pregnant? It was okay, it was harder in the respect that I couldn't always just sit down or nap when I felt shattered, because I had my daughter to still look after.
Advice... Make sure you don't say 'no' all the time to the eldest, particularly when they're showing interest in the new little one. Try and word it so it's not 'no no no' because then you might find them reacting badly to the new arrival. Encourage them to show affection and play with them etc. Try to spread your time between them both, particularly in the early days. When you've got your newborn sleeping, make sure you try and spend some quality time with the eldest. It's getting the balance right.
Hope this helps