Not a tandem rider but a long time user of good fenders. Good, deep front flaps rule. Many of the flaps that come on fenders are jokes. I regularly drill out the rivets, remove the flaps and screw on real ones that I make. If after that, the stoke still finds he/she is getting wet from the real wheel, I'd put a similar flap on the front bottom of the rear fender. (I am assuming both fenders are full fenders; projecting in front and covering the rear wheel from BB to mid height in back.