You might want to look at range of tires that you can get for each bike.
690s run Dunlop 606 I like the rear hate the front. There is always TKC 80. One typically gets about 1500 miles of of DS tires.
Motard tires are not much better. About 3000 miles for those guys.
If you go with a standard tire size ADV bike then you can get some 4 to 6000 mile tires for them.
I find that having one bike for both DS and paved road riding is all about trade offs.
If you ask me, I'd say that two wheels is all you need.