There are stoker handlebars, but whether you can use them or not depends on how high are your own saddle and her stem.
Generally, however, it seems that the most appreciated item is a suspension seatpost. Check Mark Livingood's posts on the subject (handle: thetandemgeek) because it seems there are good and bad ones. In my case, even though it would be VERY useful in our streets, I'm not there yet because my daughters are still too short.
Another improvement I made was to install two easy-to-access water bottles onto my own seatpost. A photo may be seen
here.
Finally, Ève wanted standard road bars with "fake brake" hand rests. However, since the fake brakes cost 40-50 $ and real brake levers cost 10 $, I installed the latter.