It's the parallel universeee!!!
Well, I always ride two up on the F800 GS. Actually, I'm going to Atacama Desert (Chile) in a few weeks from now and I have been around Argentina, Chile and Uruguay (three trips combined, three diferent years, two of them on little Honda cruisers). So I guess my contribution would be on the "what-you-gonna-find-that-way" side of things.
1º - To be running out of gas in Argentina or Chile you should be waaaaay of the beaten track. And you can find very good printed maps, with gas distribution in both countries. In Argentina you must look for the "Firestone Atlas", it's the best one, including very good info of Chile as well. Also, all the GPS maps provide you with good info regarding this matter. So, I wouldn't be worried about gas at this part of your trip. I've been there last dec/jan and traveled with only a 1 gallon (touratech one) attached to my right pannier. And I didn't feel I would need it. Some times a possibility of "gas-worker"s strike can drive people mad at some cities around Calafate, Rio Gallegos etc, as they start to store gasoline, make gas distribution a little more difficult. But I don't think it's a major problem, unless you have a tight schedule. It happen to me and I had to wait for a couple of hours to fill up my tank...
2º - I think you can handle the F 800 quite well, no problem. I have the same body construction and every thing is just fine. I also have the BMW's comfort seat (a bit taller than the ass killer that came with the bike
). Wife is happy with that, me too. BUT, I did some minor mods. Like lowering the forks a bit (like 2 cm) on the triple tree clamp. Added some risers (like the touratech ones, but made here in my country) to put the bars at the original position again - actually, they are a little taller now, which is great. With luggage and wife on it, the bike feels comfort and not so scary. But I'm always scanning the area before I stop the bike so I can put at least one of my feet on the ground.
3º - If I may, I would say that in this moment you should be looking for make this as much as comfortable for your wife, in order to not only make her happy, but mostly, to regain her confidence on the trip as well. Check out my bike and you will find one item that could make a good difference:
por Arjones rider
, no Flickr
see it? It's a sissy bar ( a backrest with a little bag strapped to it). Yeah, I know, it's not quite common, but hey: ask any woman riding two up what they think about this thing. You can find it here: http://www.ascycles.com/detail.aspx?ID=48324
You can also see an extra gas can on top of the pannier. It was in that city, Trellew, almost 200 km until the next gas station and people thinking the world was about to end and they should store all the gas they could. We bought the idea. And I used that gas in El Bolsón (almost the other side of the country away) to give that bloody can to someone else
So, I would go with the F 800 GS, buy a new seat, think very carefully about the pillion comfort, and have a blast!!! And don't worry about the luggage. Less is more (as they say). You can strap anything on top of your panniers at any time...