<<Some Tndems do have the ability to allow the stoker to pedal out of synch and this creates a problem for corners and Kerb crossing>>
Actually, tandems with ICS (independent coasting system) make it easier on corners and curbs, since the captain only has to be concerned with the position of his pedals and the stoker handles the position of her pedals independently. daVinci sells a line of tandems with ICS. They have pros and cons. You only get the benefit of the efforts of both riders if you pedal in sync. That said, a stronger rider can pull the team at his/her pace without being constrained by the pace of the weaker rider. Whether you ride a standard tandem where you pedal in sync or a tandem with ICS where pedaling in sync is optional, disharmony has more to do with whether you both look for ways to work as a team in light of each other's strengths and weaknesses or whether you develop resentment over each other's strengths and weaknesses. Based on your description of your wife's fear of mountain biking and trails, if you try riding a tandem, it might be considerate to start with smooth roads for a while and give her a chance to develop confidence. Being a stoker is scary for many riders. My stoker was terrified when we started riding a tandem. We ride every day and have now been riding for over 30 years. She is no longer terrified, just a bit scared when we ride. But, she enjoys it very much, perhaps in part because we very gradually increased our riding time and difficulty of terrain so she could get acclimated and gain confidence.