I think the most important consideration is that whatever you choose is comfortably rideable by BOTH of you. All too often what happens is he gets the bikes he likes and she makes do, even though the bike is, strictly speaking, bigger than she would choose for herself. (You haven't said what size you both are, so I will assume average - in other words you're a lot bigger than she is
I think some time wandering through shops with your lady doing the "test sitting" on the bikes will rule out some of the options, and perhaps narrow it down enough. One important test - can she pick it up herself unladen? Remember it will be much harder fully loaded, so if it's too hard bare, forget it.
A bike to think about is a DR350. Adequately fast, light and easy to ride, and reliable. Astrid rode hers from Alaska to Ushuaia, where we met her, and she loved it. And she's quite small. Her boyfriend had an Africa Twin, which was much heavier, and harder to ride. And it's speed was wasted.
Average speed on-road in Africa is no more than 60mph if you have any brains at all. So a big bike is a waste.
Smaller is also cheaper, and you can then get something much newer and probably more reliable, and still spend a few quid getting it well-sorted and equipped.
And yes, definitely without a shadow of a doubt have two identical bikes.
Also well worth seeing about an off-road course for both of you - I guarantee it will be the best money you spend on prepping for the trip. And it will even make you into better street riders.
Share the Dream!