'Fat' was a consideration early on, but this bike is being put together almost exclusively for Europe / Asia (and similar distance continental) runs, so essentially tarmac & gravel. In that (wide) gamut of terrains there will indeed be places where 4" up would be useful, but for 99% of the time 3" Knard is about as big as it's as it's ever likely to need to go. (If it stops on snow /sand on those, it's time to get off & push
) Another factor is that, in my experience, tandems on thin tyres can go a lot further in those conditions than 1/2 bikes - the weight & torque means you can 'dig down' and keep moving.... that has surprised not a few people in UK mudfests in the past. Last but not least going fat ads a whole bunch of issues like significant weight increase ('standard tyre' will be around the 2.5 mark), drive train geometry and (for touring much of Europe & Central Asia ) parts availability.
Value-wise fat is a non-market here even on singles (they're growing a bit on 'novelty value'). For 'specialised' tandems the market is minuscule - our Ventana is one of only 5 or six in the whole of the UK!
