It sounds like you want a bit more power (CC wise) anyway. But lessons I have learnt from two up on the AT. First off I find the AT (solo) very capable off road (it is better then i am, so it is not the bike that limits the dirt side of things).

Two up: The AT is a top heavy bike to begin with so two up un-loaded it is not great off road, loaded it can be a hard ride. BUT this is predominately due to how we pack. I know every one says low and forward for all the weight but it becomes even more important when two up off piste on a bike like this. I have found that our tank panniers (not tank bag) became indispensable, (to the point where i am custom making bigger better ones (then my Touratech bags). On road your bike can feel perfectly stable, but you get off and you will find that your front end is very light and may not do what you want. So really get the weight low. Sorry pillions (co-riders) but you are a heavy and badly packed lump when off road. So you have to compensate with packing the other stuff in the right places. .

I found that i can get away with a lot more high weight and sloppy packing when going solo. So if you can when you get your new bike, think carefully about the weight distribution of you, her, and gear.

I also am reluctant to drop tyre pressure to the same low levels that i will do solo (fear of pinch). But it does help.

Warthog is correct ride alot of bikes two-up, make sure she is comfortable (most important) and that you are too (beleive it or not less important). And most off all dont be afraid to ask her to walk durring the really bad stuff.. you get evil looks the first time, but then she see you fall and the evil look turns in o a Nelson like "HA-HA"..and she is happy that you are under the bike and not her..

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2000 Africa Twin
http://two-up-ona-twin.blogspot.com