I can confirm that the Panaracer Gravelking (slick) tires are really nice for tandem use. The hold sealant well. They are every bit as fast as a "road tire". The newer Gravelking + has extra puncture protection, but I don't know if it has extra rubber.
You might like the Simworks Volummy 38mm, Donnelly USH 40mm, or Challenge Strada Bianca 36mm as (somewhat) longer lasting alternatives to Rene Herse tires.