a little bit shorter and more upright position will be fine for the tandem versus your road bike especially at first as you get used to piloting the bike. for me i seem to have a lot more weight on my hands and arms than on my single bikes because you have to control the weight of two riders and so i run the tandem with a cm or two shorter stem and a little bit taller instead of being really stretched out like on my race bike.
there are no handling implications from the top tube differences since the tandem wheelbase is so long, it will be completely different from a single bike no matter what.