Today's "Summer Song" features a duet by two singers with a personal history that spans the decades. With Anthony Mason we'll listen in:

They'd written songs about each other, but Stephen Stills and Judy Collins had never performed together. Now, five decades after they were lovers, they're finally musical partners.

"You have no idea how much fun it is to be on stage together," Collins said.

"It's hilarious; Ma & Pa Kettle!" laughed Stills.

"No, no, we're still younger than springtime!"

Stephen Stills and Judy Collins. CBS News

Stills, now 73, and Collins, 79, are touring together this summer, after releasing their first album, "Everybody Knows," last fall.

Stills said, "I find her voice thrilling. It's like discovering a unicorn!"

They first met in the late sixties. Stills had just had a big hit with his band Buffalo Springfield.

To hear "For What It's Worth" by Buffalo Springfield, click on the video player below:



And Collins had hit the Top 10 with a Joni Mitchell song.

To hear Collins perform "Both Sides Now" from her album "Wildflowers" click on the video player below:

One night on L.A.'s Sunset Strip, Stills went to see Eric Clapton and Cream playing at the Whisky a Go Go, when he spotted Collins and her producer, David Anderle, in the corner: "They were sitting in the last booth. He said, 'Stephen, this is Judy Collins.' And I said, 'Oh, my God, I've loved everything you've done.' I kissed her hand very chivalrously.'"

The attraction at the beginning, Stills said, was "visceral."

Collins added, "We were meant to be together. I mean, there's no question about that."

Stephen Stills and Judy Collins in the 1960s. CBS News

It ignited a tumultuous two-year, on-again, off-again affair. "Sometimes I say that the rumors lasted longer than the affair," Collins said. "But the truth was that it was a very important relationship ... well, for me."

"Profound," Stills added.

"And we survived it!" she laughed.

"We were both very fairly headstrong people."

Mason asked, "So, what were the points of conflict?"

"Anything and everything, and nothing," Stills replied.

Collins was recording her next album, "Who Knows Where the Time Goes," and Stills asked to play guitar in the sessions.

After hours, alone in the studio, he cut the first demo of a song he was working on called "Suite: Judy Blue Eyes."

Playing the guitar he'd given Collins on her birthday in 1969, he sang "Suite: Judy Blue Eyes" for her. Collins recalled, "And I said, 'That's so beautiful, but it's not gonna get me back.'"

"That's a pretty compelling case, though, to put forward," Mason said.

"Yeah! No, he always said he did everything he could think of!"

The seven-minute bittersweet valentine to Collins would become a classic breakup song – and in 1969, a hit for the new band Crosby, Stills & Nash:

Tearing yourself away from me now you are free

And I am crying.

This does not mean I don't love you, I do, that's forever

Yes and for always.

I am yours, you are mine, you are what you are

You make it hard.

Hearing it on the radio, Collins said, "I was blown away by the song. The fact that it was about me was not …"

"After a while you forget that," said Stills.

"But it's pretty cool that it's about me!" she laughed. "It still is pretty cool. To this day."

WEB EXTRA VIDEO: Stephen Stills and Judy Collins on "Suite: Judy Blue Eyes"

The years passed, but they stayed in touch, and after a phone call in 1975, Collins wrote the song "Houses," about Stills and all his real estate ("Where the first fortunes went," he laughed):

When the winter finds you, you fly to where it's summer

Rooms that face the ocean, moonlight on your bed

Mermaids swift as dolphins paint the air with diamonds

You are like a seagull as you said.

Mason said, "You've both been through a lot."

"We have. We have," said Collins. "Terrible and wonderful."

Collins battled alcoholism in her early years. After coming out of rehab in 1978, she planned to visit Stills in Florida on tour with his band: "And he said, 'Don't come. You'll never stay sober down here.'

"He knew how hard I had struggled with the alcoholism. He knew how miserable I was, how terribly unhappy I was."

"And he didn't want to see it happen again?"

"He wanted to see me get a chance."

That chance came the next week when she had her first date with Louis Nelson. "Timing is everything, isn't it?" Mason said.

"Timing is everything. I mean, that was my magic moment. I always say he was part of the angel group the surrounded me, that helped get me sober."

The singer and the designer have been together for 40 years now.

Mason asked Nelson, "Are you okay with her out on tour with an old flame?"

"I don't mind," he replied. "But sometimes it rubs!"

But he's read his wife's autobiography: "So, I got to the end and turned to Judy and I said, 'Well, I guess I won!'"

Stills has a longtime partner, too: his wife, Kristen.

Stills said, "The secret of it is that we married other people. She's my best friend."

She concurred: "He gets me. And I get him."

WEB EXTRA VIDEO: Stills and Collins on surviving their affair

On the road, Judy Collins and Stephen Stills have become so comfortable that he suggested maybe they should have skipped all that old romance stuff: "And she just looked at me and said, 'No.'"

No? "Because then he never would have written 'Suite: Judy Blue Eyes,'" she explained.

WEB EXTRA VIDEO: Stills and Collins on teaming up

In the end, performing together has come very easily to them. "Oh yes, it's divine," said Collins.

"Actually, she's the best partner I've ever had," he laughed.

Strange that it took half a century to happen. But who knows where the time goes.

To hear Stephen Collins & Judy Collins perform "Handle With Care," from the album "Everybody Knows," click on the video player below:



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Story produced by Gavin Boyle.