FWIW - I have a DRZ400, and an R1200GSA - and wouldn't bother asking Susan if she'd be happy to do 100km on the DRZ. Waste of breath.
Corollary to that, two of our California Meeting organizers HAVE done some reasonable mileage on a DRZ. He rode from California to Panama, and she flew in and they rode around Panama together. They did okay with it, but rode short distances, and he put a lot of effort into a comfortable seat for her, overlapping onto the rear rack. Sheepskin, mucho padding etc. He's tall, she's not.
So it CAN be done.
Can the engine take the strain - no shortage of power, the DRZ has plenty - but it's top cruising speed at the very best is around 100kph. Yes it will go faster, but neither of you will be happy for long, event at 100. 85-90 is nice.
Yes you can gear it up, but a lot of people have fiddled with gearing, as have I, and that's my conclusion. Stock DRZ400S gearing is best all round.
The engine is also VERY robust - you'll have to work or do something stupid like run out of oil to break it.
Would I RECOMMEND is as a two up touring bike? NO!
To stay in the rough category, a DR650 would be a FAR better choice. Still good off-road, and sweet on the highway - at around 100-110kph with stock gearing, and relatively comfortable for two with an after market seat.
From there, there's loads of good two-up bikes. So your decision comes down to

1. How big are you two? Smaller is good for a DRZ!
2. How much two-up vs solo?
3. How far do you want to go two up?
4. Budget?
5. How much road versus off-road proper dirt track / single track?
6. How long do you want to keep it? For a year, anything is good to get your feet wet and help decide what you REALLY want.

Hope that helps!

__________________
Grant Johnson
Seek, and ye shall find.
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Inspiring, Informing and Connecting travellers since 1997!
www.HorizonsUnlimited.com