Thank you very much guys for all those answers.
Obviously we are young drivers, but not crazy ones, and we do really prefer to drive carefully on a road with nice landscape than to travel like a speedrace contest ! The permit in France is really hard and she just failed it once, but I really do think she drives well. But you are right, we need to gain experience (as every young drivers), that's why I was thinking that good north american road would do it, before arriving in Central and South America !
We are used to travel a lot everywhere too.
That's why, me on a better motorbike and her on a 125cc could be possible, if only the paperworks permit it (insurance, driving licence,...).
First, we'll spend a year in Canada, then see if it will be possible, I hope so.
Thank you for all the info,