For Roberto Diaz and Elissa Lee Koljonen, it was love at first chord.
The two string players — he on viola, she on violin — were both invited to a mutual friend's music festival on Cape Cod, Mass., 18 years ago.
They performed together and felt a spark "fairly quickly," Diaz said.
"We would play together and it just kind of worked out," he said in a cellphone interview while on the way to his job as president and CEO of the Curtis Institute of Music in Philadelphia.
Diaz, who performed with the Quad-City Symphony Orchestra in December 2010, will return for next weekend's concerts along with Koljonen, his wife of 16 years.
They will be featured on Mozart's "Sinfonia Concertante for Violin and Viola," a rare duet of the two instruments, and Diaz can't contain his excitement about the piece.
"It's arguably one of the best materials ever written," he said. "It's one of the greatest pieces he (Mozart) wrote, and you can imagine how privileged we feel for our particular combination to have this piece."
Diaz said he enjoys the Mozart work as much as a listener as a performer.
"It's one of those desert island pieces you could listen to forever. When you are actually in the process of performing it or working on it, you're always discovering something new, hearing something differently," he said. "It's a piece that never gets old."
But the piece is not without its challenges.
"Playing the Mozart is very tough because it's so exposed, so clear, if you will," Diaz continued. "It's so organic and it has to sound so incredibly natural, yet it is not easy to do that and have it come off as if it's the most obvious thing."
Diaz, 53, and Koljonen, 40 — the parents of a daughter, 11, and a son, 7 — both have busy careers as soloists.
"There's ups and downs" to having a duet partner who is also a life partner, he said.
"Knowing somebody that well and playing with them a lot gives you a certain comfort level you can't find anywhere else," he added. "That's a great thing, obviously.
"We get along very well. There's mutual respect musically and personally. I'm not going to tell you it's without disagreement that we should play it this way or we should play it that way," he continued. "But there's always the understanding that this is definitely not going to be the last time we play this."