Fabbee Lady,
In addition to my response to your other post:
I was not suggesting you keep 2 colonies in the one hive; exactly the opposite really - you would require two hives, at twice the expense!
There is not so much difference in the dimensions of the two variants. The Dartington roof can be made, as mine, in two pieces (mine are actually joined but could easily be used as separate halves).
The four hives I was referring to (for the cost of a beehaus) were to include flat-pack seconds as frames and that seems not to to your liking. I would not recommend some of the polyhives on the market. Hivemakers suggestion is a good one.
There are far more local issues than you can envisage, as a non-beekeeper. Those local beeks will have a wealth of experience and you need to listen to them carefully.
My advice at this instant would lean toward a complete polyhive or wood equivalent and the bare necessities for swarming, unless I was aquainted with all the relevant issues for your particular situation. If the predominant hive in your area is National, polynationals are not so common and decent sized ones more than a premium!
I am confident your local beeks would also recommend expansion to two colonies (at least) if at all possible, so your use of all the available space with one large hive may be flawed. No easy decisions - and it is your call.
I could measure and weigh component parts if you would like, for comparison. Pm me if you do.
Regards, RAB