WARREN — Emily Saliers and Amy Ray have been proving the “Power of Two” as the Indigo Girls for nearly 40 years.

The way their voices blend and become something far greater than the individual components, it’s like a logic-defying math equation — 1 + 1 = 37. And that power was on full display Friday for a near-capacity crowd at Packard Music Hall.

“Power of Two” is a frequent setlist choice (the duo has performed it more than 1,000 times according to setlist.fm), but it took on a special role on Friday. It was a special request and followed a proposal in the audience between Holly and Amanda. I’m not sure which one proposed, but the recipient said yes (certainly would have cast a pall on the song if the answer was no).

The 22-song, 105-minute set, performed in front of a backdrop that looked like shelves stacked with (banned?) books, was structured similar to an old Grateful Dead show when the band would go back and forth between a Jerry Garcia lead vocal and a Bob Weir one.

Ray’s vocals sounded particularly strong on songs like “The Wonder Song” and “Are You on Fire.” Saliers’ voice was a bit more soft and fragile on lead vocals for songs such as “When We Were Writers” and “Least Complicated,” but both singers’ tunes hit another level when those harmonies took over.

Ray and Saliers did have some help. Violinist Lyris Hung had her own showcase mid set, recording multiple loops and playing over them to create a solo symphony of sound. As impressive as that was, the colors she added to the arrangements of “Gone Again,” “Shame on You” and others were better.

Opening act Lucy Wainwright Roche, whose 45-minute set featured as much self-deprecating humor and Q&A as it did actual music, contributed backing vocals, acoustic guitar and (on one song) kazoo.

Ray switched between acoustic guitar and mandolin, while Saliers played mostly acoustic and some electric guitar and showed off a few dexterous leads, particularly on “Chickenman.”

Then there was the choir of nearly 2,000 voices that frequently sang along. I gotta say, I’ve heard a lot of audience singalongs, but the quality of the vocals among Indigo Girls fans was top notch. Instead of detracting from the concert, those moments enhanced it.

Not surprisingly, “Closer to Fine” — an already popular Indigo Girls’ song that got a boost from last year’s “Barbie” movie — got the biggest reaction, but this clearly wasn’t a crowd of Indigo Girls’ bandwagon jumpers.

When Roche asked the audience where they were from, shouts of Pittsburgh, Cleveland, Akron and Columbus could be heard along with Mahoning Valley towns.

The crowd was roused by a passionate and powerful “Go,” the undeniably catchy “Shame on You” and the beautiful finale “Galileo” as it was the “Barbie” song.