Have you seen the ABC Adventure Buggy? Its like the mountain buggy (also manufactured in New Zealand), is the width of a single pram, but the toddler seat goes up the top facing Mum - so neither childs view is blocked by the other. They also have a new infant/toddler reclining toddler seat, so that the newborn can face Mum at eye level and recline when sleepy (comes complete with hood & rain cover etc), and the toddler can face outwards down the bottom. They also have a front mounted toddler seat if you prefer this - although from reviews I believe this option is harder to steer.
The pram is traditionally a 3 wheeler - but when you buy the package with the additional seat they recommend (and I'm pretty sure include) an additional 4th wheel with adapter bar for extra stability (they have pics of all this with video demos on their website).
Go bubba go is the Australian distributor of this pram- unfortunately its not cheap but top quality!!
If your looking for something a bit less pricey, I know the love n care Uni Coupe takes a front mounted toddler seat.
Goodluck.
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