What happens when you put two choirs in a room together? They sing. That, at least, is what happened on a chilly winter night in February, when the members of the all-male San Francisco vocal group Chanticleer and their Twin Cities counterparts Cantus capped an evening of hanging out together at a bar in Virginia.

The meetup didn't happen by accident. "The two ensembles have known of each other for a long time," explained Paul Scholtz, a tenor in Cantus. "And when Cantus first began, I think their original members were inspired by Chanticleer. We've even had members audition for both groups, and move from one group to the other."

San Francisco is a long way from Minneapolis, however, and despite scoping one another from a respectful distance, the two choirs had never managed a face-to-face meeting. Until, that is, Cantus bass Chris Foss began forensically tracking where each group went on tour, and whether there was any geographical overlap.

There was. On Feb. 2, 2016, Chanticleer had a booking in Newport News, Va. By driving all day after a morning master class in North Carolina, it was theoretically possible for Cantus to make the hookup. But would it actually happen?

"The trouble with tours," said Scholtz, "is that you're always so exhausted. In your off time you sometimes just want to go to bed, and I'm certainly guilty of that. We would have no free time if we did this, so the group had to vote whether or not we wanted to try and make this concert."

They voted yes, drove seven hours to the East Coast and, as Scholtz remembered, "snuck in during the first half of Chanticleer's show." For the Cantus men, the journey was well worth it. "It was really fun to see how they structure their program; it's like you're seeing a mirror of yourself on stage. I'm sure it was kind of intimidating to have your peers in the audience. It was so gracious of them to have us."

What happened next earned itself a small place in internet history. At the Cove Tavern near Chanticleer's hotel, as the two groups shared post-concert refreshments, somebody suggested an impromptu spot of vocalizing.

"We were getting up, ready to go," recalled Scholtz. "But everyone wanted to do it right away, it was a total buzz." The "Ave Maria" of German composer Franz Biebl, a classic of unaccompanied choral music, was the chosen piece. An iPhone recorded the performance. The resulting video has since gone viral, amassing nearly 1.5 million views on Facebook alone.

How on earth did that happen? Scholtz isn't quite certain. "It was nuts," he said. "I think what's special is you have these two groups, and you can imagine you have different turf, manufacture a rivalry, whatever. But in our current world, our political climate, when you see folks coming together like that, I think it's such a powerful way to respond to that us-and-them situation, as opposed to there being any cattiness."

Adam Ward, an alto with Chanticleer, agreed that the meeting of what he called the "two fraternities" was "warm and touching." Then again, he blanches a little at the memory because "in the video you have people praying to the Virgin Mary holding their pint. And you're wondering, is that appropriate?"

Appropriate or not, the beautifully blended, moving performance of the "Ave Maria" quickly captured the virtual world's imagination and springboarded the idea of a concert, bringing the 21 voices of Cantus and Chanticleer together on a single platform.

The Oct. 3 event is undoubtedly the highlight of the fall choral season. Ward, for one, can't wait to relive the sound the two ensembles make in combination. "It's a historic moment for both groups," he enthused, "and we'll get the opportunity to really go into depth preparing the pieces that we do together."

The performers won't be holding beers or smartphones that evening, but it's a solid bet that Biebl's "Ave Maria" will be revisited, and that choral followers are in for a unique, possibly once-in-a-lifetime experience.

Cantus' Scholtz certainly thinks so. "One of the reasons this concert's going to be so special is that as singers, we're always in competition with one another. You're always auditioning for that one spot in the choir, that one solo with the orchestra. But in this concert we're just singing together, with no other motive except the joy of community and the joy of singing."

A recent transplant from Ireland, Terry Blain is a Twin Cities music and theater writer.

Cantus and Chanicleer
When: 7:30 p.m. Oct 3
Tickets: $30-$250, 612-371-5656, minnesotaorchestra.org