2 1/4 years between mine. If you walk a lot you really need a double, probably a Phil& Ted's as - as others have said - there are times when you are in a hurry/in the supermarket and you need them both strapped in and ready to go, go, go!
Having said that dd2 HATED the cocoon thing on the p and t and screamed the place down every time she was put in it, huge relief when at 4 months she could sit up in it. Also, and a lot of people have this problem it keeps getting punctures, which is a pain in the butt. During first puncture I went out and bought a buggy board for the old Maclaren and now if we're just going down the road to the playground or whatever I use that combo. I hoped, like you, that dd1 would walk but they tend to regress when dc2 arrives and get jealous of little baby all cosy in the buggy, at least my little devil did! It all has cost me far, far more money than I like to think but in the end needed both options. Grrr