Well, when most of the time you would only use one of the two, what's wrong with the logic? I hope to get a 1 and 1/2 set - 95+% of the time, I'll just use the half bit but when I have all the extended family round for dinner, I will need the bigger one instead of or as well as.
Using a 900 wide oven to do cheese on toast or grill one bit of steak or heat up a single serve of homemade pizza seems a bit of an overkill.