Check out www.usacycling.org
and search for local races. Many of the time trials have a tandem category. Usually, there are local unsanctioned practice time trials that will allow tandems to race. Finally, there is the state / district TT championships which always have a tandem division by age.
Whatever you can do, or dream you can, begin it. Boldness has genius, power and magic in it. - Johann Wolfgang von Goethe, writer
"Change is easy. Improvement is far more difficult." - Dr. Ferdinand Porsche