I don't particularly like the baby underneath the toddler style of the phil and ted's from the brief looks I've had - my then 1yo DD refused to go in the bottom when a couple of friends of ours were trying their's out: they had a nearly 2yo DS and their DD had been born at 24+wks so definitely needed a double but didn't have a second to go in it quite yet iyswim. DD hated being underneath, but she wasn't a newborn, so maybe its easier if they're used to it?
Unfortunately, I think most doubles that aren't side by side are like that though. My friend has a Jane one that she got on the Preloved website, which seems pretty good, and has them in a line, rather on top of each other. Think that model is £500+ new though and you need a new car seat to use on it etc. so it all starts adding up. In an ideal world, I would keep my car seat, but the pram we have (a Britax, which I love!) has a britax car seat and isn't useful on others. Other ones you can obviously have the maxi cosi or whatever, which is transferrable.
My sister borrowed a friend's double buggy the other week to go shopping with her DS who's 4 and with my DD (2) so I guess it'll be useful for quite a while regardless. It is a bit of a pet peeve of mine though, when I see 2 parents pushing a nearly school age child around a shopping centre - I always think why aren't they walking?!!! Its not like you've got a gaggle of kids to handle - 1 pre-schooler is easy! Especially if they have a dummy in too!!! But's that probably just me...