If you're dead set on the option to have a second seat and prefer tandems I've definitely noticed that the baby jogger city select seems to be the most popular. I can't imagine it being that easy to put up and down but don't know how hard the phil and teds is either. Watch some you tube vids & compare.
Problem with tandems is they can be harder to push though FYI but you can't exactly convert a single to a side by side so I imagine they are out for you.
I know valco prams have a toddler seat that kind of sits suspended on top.