I had just under 19 months between my first two, and just under 16 months between #2 and #3. There will be around 16 months between #3 and #4.

I didn't buy a double pram for any of them, I have a few good carriers that I use for one and push the other. So thats an option too. I got a Maclaren stroller because it can adapt from newborn to toddler, so I use that for either the baby or the toddler, and then my 3.5 year old walks. Sometimes the toddler walks too and I carry/push the baby.

But I've never really had a need to get one, so there is no rule to say that just cos you have them close together you have to buy a double pram, they are so bulky and heavy (most of them anyway!) So thats another option too - fork out the dollars on a good carrier instead!