Travelling with two toddlers: single or double stroller?

When we had our first child we bought a Baby Jogger City Mini GT stroller. And we loved it. Then when our second child came along just 18 months later we invested in almost the double version, the Baby Jogger City Mini double stroller (no GT - some small differences but basically the non-GT suited us fine and was both a) more available and b) less expensive). And we loved it even more - me especially, as it was when I was on my own with my two little darlings that I most appreciated being able to contain them both in one space! Seriously though, it meant I could take them both out for long walks (longer than I could comfortably manage with the baby permanently in the carrier); if my older one did want to walk for a bit I could always quickly strap him into the buggy around busy roads; and if they ever both fell asleep at the same time it meant I could park them up and enjoy a bit of hands-free time (and take some cute photos!). We chose a side-by-side stroller after talkin...