Good questions Skierx -- the way ours is laid out, yes combining the two living rooms you would have two hallways of rooms, and there would be a spare kitchen. We would probably convert the kitchen into another room like a study room, or just open up those walls too (for sure it's not structural, the wall is super thin).
We paid a (relatively) low price for our current flat and the next door would be at the current market price, but the blended average is still lower priced than buying a flat that is double sized (and now paying the fees and taxes for selling and buying at higher transaction cost). Plus I suppose we are also guilty of having a bit of inertia to not move out due to laziness and also we really like our little community here.
As for less liquidity mentioned by jrkob, that is a good point, I hadn't thought of that specifically, we would plan to live here for another decade then, if we do this. But it's a very fair point to consider, we would probably be the "biggest" unit in the estate and the demand would be limited, few would expect a unit this size here. If this place was full of duplex units then it could be expected to have more deal flow in this size...
Thanks for all the thoughts! More for me to think about...