The two houses
can be combined as most of the parts are the same in both sets. But as I think you realise, you will end up with a doorway to nowhere on the highest floor because the only triple-width piece you will have for the front wall has 2 windows and a door!
You can make a triple width wall with three windows from a double width wall (2 windows) and a single width wall (1 window). The single width wall only came in set
7411, which was an extension floor for the Victorian mansion. The part was
30660240. 7411 is hard to come by now, but it seems to be due for re-release later this year. See
here. So you might get the single wall as a spare part later this year (act fast before they sell out!).
Another way to get a single-width window wall is to make one from a double or triple wall. This is probably the cheapest option. It involves sawing and glueing pieces of a wall together. You would probably get a suitable wall to modify like this on eBay. I have done this in the past and I could hunt out one of the walls I created to show you, if you are interested.