On our T50 tandem, we're running 54-44-26 with 11-34 9 speed, SRAM X.9 long cage RD, with no issues.
Tandems need wider range gears than singles; and 54/11 is used on flats with a tail wind or slight down grades.
To determine chain length, you need not only the sprocket teeth count, but also the dimension from the center of the BB to the center of the rear axle.