Jellycow, if you are going to tour like the average European couple, then the R80RT will do you fine. If you plan to tour like the average American couple and want to take everythign inculding the kitchen sink, then an R100RT is still not enough for you.
If it was me, I would go for the R80 and see how it goes. They are the smoothest running boxer built and have just about as much power as the R100. All they really need for 2 up touring is some upgraded suspension, i.e. good quality heavy duty shocks and springs on the back and possibly upgraded springs inthe forks and slightly heavier oil. They get good gas mileage when ridden properly and can get down right poor mileage if you are trying to ride it like a crotch rocket. Normal gas mileage forthe R80 should be in the range of 45-50MPG and some have been known to get as high as 55MPG (or so I have been told), while the R100 will usually top out at around 45MPG. Thr R80 does have lower horsepower, but with the gearing it has, you really don't notice it much, maybe on a high speed pass up hill.
For touring you will want reliablity. The stock R80RT will do what you need to do and get you back with no issues.