Stoker Kay rides a Serfas Cosmos saddle, currently has 18,000+ miles on it.
However, everyone's buns are different, some folks ride more upright than others (especially on rear of tandem) and some stoker use a shock absorbing seatpost.
A very soft, squishy saddle is not always the answer! What feels good for the first 5 miles may not do as well after 30 miles. So try as many saddles as you can 'til stoker smiles!
Pedal on TWOgether!
Rudy and Kay/zonatandem