The duo’s “Moon” shone once again during a return engagement in 2001, and now they’re back to open ACT’s season with a victory lap they like to call “Old Hats.” Whether they’re performing hat tricks, ribbing each other or performing a good, old-fashioned vaudeville dance, they do everything with a polish and precision that makes everything look natural and easy. For this show, directed with verve by Tina Landau, they have a special weapon in the person of Shaina Taub, who contributes original songs and plays with the four-piece band. Shiner, whose style has always been a little more bitter to Irwin’s sweet, has a particular genius for working with audience members, and a has-been magician routine in Act 1 only whets the appetite for the feast that comes in Act 2. [...] they break the silence in a bit with Taub where they sing show tunes, quote “Hamlet” and take a quick (and wonderful) trip through the voices of “The Wizard of Oz” before tantalizing us with a taste of “Who’s on First?”