Siblings have always played music together. The history of the last hundred years, admittedly because it's been so well documented, is full of examples - the Everlys, Beach Boys, Kinks, Oasis, numerous country and bluegrass outfits - of brothers and sisters in harmony or, in some cases, disharmony.

Something special seems to happen in particular when twins get together. Take Ali and Mike Vass, rising stars of the Scottish traditional music scene, who kicked off their first major tour this week as part of the Scottish Arts Council's latest TuneUp package, Fiddle Rendezvous.

There's an electricity between this pianist/singer and fiddler from Nairn when they play as a duo that working with other musicians doesn't always bring out. When Mike appeared in the final of the Young Scottish Traditional Musician of the Year in 2007, for instance, good as the supporting musicians always are, he just couldn't quite create the energy that playing to his sister's piano accompaniment almost routinely sparks. Or have I got this wrong?

"No, it's something that we're aware of, too," says Mike. "In fact, I find playing with Ali totally different from playing with other people. It's probably because we've been playing together so long and we know each other so well, that we can totally relax and at the same time push each other. I've noticed it especially recently when we've maybe not seen each other for a couple of months and we get back together for a gig, and it's there instantly."

It's now more than half their lifetimes ago since the twins, who are in their early twenties, made their live debut as a duo. They were 11 years old and were given a platform at their mother's aunt's golden wedding anniversary.

"We played for about 30 or 40 minutes, which is quite a lot for eleven-year-olds," says Mike. "But we practised for weeks beforehand. It was quite a big thing for us."

Bigger things weren't so far off. Three years later, in 1998, and by this time joined by their younger sister and brother, the Vasses supported banjo whizz Alison Brown's group at Celtic Connections, receiving lavish critical praise in the process.

"We actually played quite a bit as a quartet until the other two lost interest and got into other things," says Mike.

Mike and Ali, being twins, were always close and sometimes close rivals. Born in Inverness, they lived in Durness and then spent three years in Duns, in the Borders, before their father's work as a primary school headmaster took them back north to Nairn. By this time they'd had their first falling out through musical differences. At the age of six they both started playing piano.

"It was more of a miniature keyboard," Mike remembers. "But Ali was better than me within about two weeks, so that was the end of music for me until, a couple of years later, I got a violin. Our uncle had played violin with the orchestra of Scottish Opera and I thought that was quite cool, so he was my inspiration."

Classical violin lessons at school were quickly supplemented by learning Scottish tunes at home. With fiddle tuition from Ian Hardie, formerly of Jock Tamson's Bairns, Mike began competing successfully at various music festivals, including the National Mod. Between the ages of 11 and 14, he won two Mod gold medals and was voted Most Promising Musician at Oban Music Festival. He also played with the Highland Region Youth Orchestra, alongside Young Scottish Traditional Musician of the Year 2003, Anna Massie, and the National Youth Orchestra of Scotland, before going on to teach fiddle at Balnain House, the former centre for traditional music in Inverness.

Meanwhile, Ali had continued on piano, had won the Best Instrumentalist title at Inverness Music Festival and would go on to study traditional music at Newcastle University for a year, in between the philosophy degree she already has and the law degree she was sitting exams toward last week. Hence the long separations.

While recognising the value in having a lawyer in the partnership to check contracts and other legal niceties, Mike is hoping that Fiddle Rendezvous will prove educational as well, and give the duo exposure to a new audience in bigger venues than they're used to. A meeting of Irish, American and Scots fiddle styles, complemented by Gaelic and Scots song, Fiddle Rendezvous finds the Vasses in the company of the brilliant Dundalk fiddler Gerry O'Connor, his accompanist, Breton guitarist Gilles le Bigot, and the duo of American old-time master Bruce Molsky and Gaelic singer-harpist Mary Ann Kennedy.

"Bruce has promised to teach me how to sing and play fiddle simultaneously, Appalachian style, not that I can sing at all, and having been playing guitar for three years now, I'm hoping to pick up some tips from Gilles," says Mike. "But just being around these four much more experienced musicians, who are all at the top of their game, is a thrill for us. We did a preview concert at Celtic Connections and it was a real treat, although Gerry, who knows loads of tunes, gave Bruce and myself this mad jig to play called The Kitten - and it was quite reassuring that it took Bruce about as long to get it as it took me."

As with the preview concert, the Fiddle Rendezvous format gives all three duos time in the spotlight, with the six musicians then forming a band to explore songs and tunes that have travelled between their respective countries.

"Playing in a bigger group is fun, especially one where there are so many contrasting styles," says Mike. "But the real attraction of playing in a duo is that there's less to go wrong. You're more exposed in a way, too, and so that puts more pressure on you, and you can't hide."

What about sibling rivalry, though, do they ever have a Phil and Don Everly or a Ray and Dave Davies moment?

"Well, Ali likes to be right all the time and she's just that little bit older, so she pulls rank quite often," Mike says without sounding entirely convincing. "But generally we get on pretty well. We both have to like a tune before we play it or a song before Ali sings it, and we'll spend quite a long time working on something until we're happy with it. The main thing is, though, I really enjoy playing with her."