Janeite, that is really interesting - it gives me hope that we may be fine with a single ... though like nanoo, I currently feel I'd be lost without a buggy option for DS whenever we have to go faster than dawdling pace.
Nanoo, I hope you don't mind, but as we have similar ishoos with this I am now sort of using your thread as a place to think through our options too! Hope it stays relevant ... if not, apologies for semi-hijacking
So: on my optimistic days I am thinking we can manage with the Swift (it is said to be suitable from birth even without the carrycot), because DS attends nursery three days a week and DP is off work at the weekends. So I will only be managing both of them alone for 2 days a week, so with the Swift and a buggy board we might get by. If we do this, I will also probably get an Ergo sling so that if DS is really desperate to ride for some reason, he can and the baby can go in the sling. (Other threads here have told me that the Ergo is really comfy for big babies, and I'm assuming this new DC may also be big -- last time we had a Baby Bjorn Active and none of us, including DS, liked it much.)
Also, if we can afford it or the PIL want to help us, there is now a Swift carrycot which we could invest in to use on the days I am alone with DC2. It might make things comfier for the baby and I doubt it would see really heavy use, so we could prob sell it on afterwards.
On my less optimistic days, when I think about all the friends who've told me they tried to manage without but ended up desperate for a double buggy after all ... then I'm thinking of a second-hand MB Urban Double. There seem to be a lot of them on eBay and because MBs seem to be so incredibly solid, I would be happy buying used again from a seller with good feedback. We might even manage to sell on again if we're careful; I know there are other passionate MB fans out there, so they hold value quite well.
I change my mind on this approximately three times a week.