When the surviving members of the Grateful Dead assembled before stadium masses in the mecca of Chicago for their Fare Thee Well swan song in 2015, it certainly wasn’t the last chance to see those musicians tap into that hallowed songbook. Three-quarters of those core survivors have struck perhaps their most lucrative endeavor yet with Dead and Company, filling out the ranks with younger bucks like John Mayer, whose musical acumen nearly outweighs his odd celebrity.

What’s been missing of course is Dead bassist Phil Lesh, who followed Fare Thee Well with a semi-retirement deal where he primarily plays one venue in the Bay Area and one in New York. This week offered Lesh’s first Boston visit since 2013, when he last played town with singer/guitarist Bob Weir in their band Furthur.

The occasion was Weir and Lesh’s first-ever duo tour, a chance to hear those two Dead principals in a more intimate, stripped-down setting—a practical conversation, warts and all (like any good jam) between their stringed instruments and rougher voices.

“Woah-oh, what I want to know is, how does that song go?” Weir and Lesh sang together in “Uncle John’s Band,” the opening number to Thursday’s second of two sold-out nights at the Boch Center Wang Theatre. In the case at hand, sure they knew, having played that song hundreds of times. But they wandered off track a bit in a jammed mid-section, as the duo thing offered a more primitive tightrope walk, variously seeming both trepidatious and transcendent.

When they followed with “Black Throated Wind,” at least Weir’s full-throated affirmation dashed any of the previous night’s questions over the shape of his voice or (the lack of) energy in his pacing. And while he muffed some words in “Minglewood Blues,” where he wielded a slide on acoustic guitar, Weir managed to navigate Bob Dylan’s “Girl From the North Country” quite seamlessly.

Lesh’s meaty tone on six-string bass as he lent spidery counterpoint and virtual leads only made up for the absence of a second guitar, keyboards or drums to a point. But he and Weir have taken to guests on the three-city tour, starting with percussionist Wally Ingram, who often painted shadowy drum/cymbal accents.

The main guest action has developed for second sets, with ex-Dylan/Levon Helm guitarist Larry Campbell and singer Teresa Williams aboard for both Boston nights. They missed the firepower of Phish’s Trey Anastasio, who revised his Fare Thee Well role in New York the previous Saturday, but brought sure-footed depth and versatility to the Americana styles of the Dead repertoire.

Weir and Campbell gnashed on dual electric guitars through “Jack Straw,” though much of Thursday’s second set tilted to acoustic input on early Dead nuggets. Campbell sawed away on fiddle in “Cassidy” (which went so far out in a jam that Williams had to jump forward when Weir suddenly returned to vocals) and “Viola Lee Blues.” He rode an interesting if meandering “Mountains of the Moon” on bouzouki and lent crisp acoustic guitar to a spry “Cumberland Blues” and a mighty set-closing “Morning Dew,” evoking memories of ’70s Dead shows at the theater back when it was called the Music Hall.

After a tour wrap-up in Chicago (perhaps with the rumored Mayer cameo that Boston missed), Weir will be back with drummers Bill Kreutzmann and Mickey Hart for a Dead and Company summer tour that starts at the Xfinity Center on May 30. Hopefully Lesh—who, at 77, is Weir’s senior by seven years—makes it back to Boston again soon, but at least he left his long-overdue signature here this week.