Yes, more detail about your riding scenario will help. Are you trying to ride it alone at times? Tandem wheelbase length could be an issue if you are going solo. Also, if you and the other rider are not leaning into turns correctly with each other, then this can be causing skid/balance issues.
One last thing - the heavier rider may need to be in front. The front wheel is all your stopping power and so their weight will stablize the front end. If there is a great deal of weight difference in the riders, then you will have to experiment which position allows most comfort and handling.