It was only when he went up to New York, to catch Marie Collier's US debut in Tosca at the Brooklyn Academy, and saw the same face twice over in the box-office queue, that he finally put two and two together and realised they were twins.
It was a prophetic encounter. Epstein is now general director of WNO, and the Alden brothers are currently causing confused double-takes up and down the corridors of the company's John Street headquarters as they rehearse their respective productions of Puccini's Turandot and Handel's Ariodante.
The twins tend not to cross paths much these days, having effectively carved up the operatic world between them: Christopher gets America, David the rest. It avoids confusion. Not that their production styles are as identical as their looks, but there is a distinct family resemblance. After all, they have shared the same background, upbringing, education and influences.
Born in 1949 into a New York theatrical family - their father an actor turned writer (for TV, movies, theatre), their mother a ballerina-cum-chorus-girl (in the original Broadway productions of Annie Get Your Gun and On the Town) turned teacher - they both studied violin for years before discovering opera in their mid-teens, as standees during the final season of the old Met before its move to Lincoln Center in 1966. After that they were hooked. Although they both went briefly into acting after leaving Pennsylvania U - significantly, while Christopher was in a Broadway musical version of The Two Gentlemen of Verona, David was in an off-Broadway Waiting for Godot - they were both waiting their chance to move into opera.
So far David has made the most impact over here. Ever since his UK debut with an infamous 'punk' Rigoletto for Scottish Opera back in 1979, he's been hitting the headlines and dividing his audiences. His series of shows for ENO began with the notorious 1984 'chainsaw massacre' Mazeppa that reputedly had members of the public puking in the aisles, and led one critic to dub the director the 'Sam Peckinpah of opera', and went on to include a 1989 Masked Ball that ended in a riot of boos and bravos and made front-page news in the London Evening Standard ('Night of uproar at the opera') and Today ('The Masked Brawl'). David Alden's name soon became synonymous with the whole so-called 'conceptualist' school of opera production.
Based mainly in America, Christopher, though technically the elder ('by six minutes or something'), has been slower to make a mark on Europe: the WNO Turandot is his British stage debut. The time-lag may owe something to his only previous work in England, a South Bank platform performance of the Brecht / Weill Seven Deadly Sins which he directed for the London Sinfonietta's 20th anniversary. It was, he admits, a catastrophe. 'It was a big celebratory occasion and, um, I just went for it and did a full-out, rather edgy and ugly sort of little production of it and, um, no one seemed to be too pleased by it.'
Working in America, he feels, also stunted his growth towards a personal approach. 'There, you're called a maverick if you work in that style, and you don't get hired. Whereas here, the maverick thing was encouraged, at least back in the Seventies and Eighties, even if, by now, it's a little . . . the tide seems to have turned somewhat.'
Reports of David's doings didn't exactly help. 'I've lost a lot of jobs, I'm sure, because of David Alden,' he laughs, 'and I hope he's lost some because of me]' Not that he's done badly: he opened the season in San Francisco with Verdi's I Vespri Siciliani and has worked at most other major US houses. He even runs his own company, Long Beach Opera, in Southern California.
Interviewing the two brothers one by one in the dress circle bar at the New Theatre, Cardiff, it's hard to suppress the suspicion, when Christopher leaves to send his brother up from the stalls, that he hasn't just done a quick-change into a fresh cardy and come straight back. But, looks apart, they're very different customers - Christopher more relaxed and open; David spikier, tenser, more defensive.
When I mention the WNO rumour that they have been taking one another's rehearsals, both deny the charge. But while David appears genuinely shocked ('No] Why? One of us is plenty really'), Christopher seems quite taken by the idea: 'No, but we should have done,' he says mischievously, 'because we were rehearsing right across the hall from each other. But when you're rehearsing, each moment is so precious that I guess we didn't get around to it.'
Not that they'd get away with it. You might not know who was who if you ran into one of them in the corridor, but you couldn't confuse their work on stage. As Epstein says, their styles are 'shockingly similar yet totally different'.
Received opinion has it that David's work is edgier, more abrasive, more extreme in its use of symbols and the primacy of its design, while Christopher's is seen as softer, more lyrical, perhaps more concerned with the depth of the performances. Christopher himself disagrees. 'I mean, these are just matters of degree. I'm just as concerned with design in my own way, I'm sure, but I'm somewhat less known for the violent and brutal style than my brother is. I don't think I've pushed that quite as far as David has.'
Whether either one is more suited to any particular opera, Epstein is not sure. Matching man and music is, for him, a matter of instinct. 'I can't explain it. But if I were doing the Ring with them both, I would use David for Rheingold and Siegfried - the darker, edgier pieces - and Christopher for Walkure and Gotterdammerung. And if I were doing The Trojans, I would get David to do Troy and Christopher to do Carthage.'
Getting the twins to work side by side is just about possible. Getting them to work together is far from it. The one time they tried - co-producing the Mozart / Da Ponte trilogy for Daniel Barenboim in Chicago in 1992 - David took Don Giovanni, Christopher took Figaro and they were meant to do Cos together. As Epstein says, 'It seemed like an amusing idea to have two brothers do this piece about two sisters. But in the event they fell into this interesting pattern where one would stage it and the other would just sit and watch. I don't think they could actually work together. They each have their own vision and it needs one person to communicate it.' Funnily enough, each now lists the Mozart trilogy on his current CV, but neither acknowledges the other's collaboration.
A touch of sibling rivalry perhaps? 'I'm sure there's a lot of healthy - or not healthy - rivalry between us,' Christopher concedes. 'I'm sure that's one of the things that pushed us both to make careers kind of early on . . . But, er, it's kind of kept healthily under wraps - as long as we're both doing the kind of work we like to do.'
Christopher's Turandot might prove a provocation. 'Why? Because it's the perfect David Alden opera]' says David wickedly. He's only watched a few minutes of his brother's rehearsals, but he already knows it's 'absolutely how I wouldn't do it. I understand what he's doing and some of it is very stylistically similar to what I do. But he's doing it more coolly than I would do it. Mine would have more sort of overt violence or something about it. Now, which is better for the piece I'm not sure . . .'
'Turandot' opens tonight, 'Ariodante' opens 24 Feb, Cardiff New Theatre (0222 394844) and on tour