Bluegrass is usually thought of as a folk idiom. Jazz, on the other hand, is typically considered a music of innovation. Bluegrass is traditional and backward-looking hill music made by people who started scraping away at that one bow out in some godforsaken holler in Ireland, kept it up all the way across the Atlantic, and didn’t stop even after they’d crawled up into some godforsaken holler in Kentucky. Jazz, on the other hand, started metastasizing fecundly as soon as it was spawned, sprouting an evolutionary tree that connects King Oliver to Louis Armstrong to Duke Ellington to Miles Davis to John Coltrane to Ornette Coleman to the scrawling noise of John Zorn.

There’s some truth in those narratives, but only some. If you wanted to tell the story differently, you could see bluegrass and jazz not as backward looking and forward looking, but rather as two branches of a single American tradition, both inspired by the blues, improvisation, the popular music of the 20s and 30s, and the freedoms and anxieties of rural people moving to the cities.

The new album American Legacies, a collaboration between the Preservation Hall Jazz Band and Del McCoury‘s bluegrass band, attempts to make a case for the second story. And in many ways it’s convincing—certainly the music is often seamless. “Shoeshine Blues” swings jovially, the bluegrass strings fitting neatly into the quacking horns on the chorus and the horns and banjo and fiddle all taking their turns on the solos. It’s a little bit Django Reinhardt and a little bit western swing—comforting in its familiarity.

The same could be said for a lot of the tracks. McCoury gives “Jambalaya” a straight down-home vocal reading while the PHJB chugs swampily along before erupting into King Oliver-style solos. “Mullensburg Joys,” a jazz standard by Jelly Roll Morton that bluegrass pioneer Bill Monroe covered with a different title, is done via handoff: the PHJB starts it off as pure New Orleans jazz, breaks for Del McCoury to play some bluegrass . . . and then moves into a horn solo over bluegrass backing. Mashup artists, eat your hearts out.

All of which makes this sound like a successful outing. And I suppose it is, sort of. But while the ease of the set is in some ways its biggest charm, over the course of the album—or sometimes even over the course of a single song—it wears out its welcome. It’s true that bluegrass and jazz have many common roots. It’s true that they have a good deal of repertoire in common. Still, though, isn’t this just a little too relaxed? When Bob Wills, the king of western swing, mixed jazz and folk, it sounded right, but it also sounded more than a little weird and wired. Wills would let rip with a crazed “aaa-haah!” while some nut tried to make his rudimentary electric guitar sound like Jack Teagarden’s trombone or played dirty blues licks on a country hoedown. At times, with Wills, it sounded like everybody in the band had gotten a different memo. Were they country? Were they jazz? Were they blues? Were they traditionalists, were they forward looking, or were they just plain pop? Who cared? Wills yelped, and different musical traditions clanked into each other with sublimely offhand chutzpah.

That’s the sign of a living music, I think, and both old bluegrass and trad jazz had their moments of heterogenous brilliance. In “Irish Black Bottom,” Louis Armstrong declared with a hyena laugh that he was born in Ireland. Bill Monroe, in a flash of rather desperate genius, performed a quasi-rockabilly take on Elvis’s version of “Blue Moon of Kentucky,” a Monroe tune that had been a hit for him seven years before the King recorded it.

At such moments, there was a sense that genre and demographic boundaries were there to kick over and stumble through rather than to lean against. Jazz could come out of Ireland; bluegrass could race ahead to rock. With a large and fickle popular audience to keep entertained, jazz and bluegrass in their day were making it up as they went along. In the absence of a safety net, there was an edge to both traditionalism and innovation—a sense that each was a strategy chosen moment by moment and clung to for dear life, for cheap thrills, or for some combination thereof.

So as it turns out, the problem with this meeting of Del McCoury and the Preservation Hall Jazz Band isn’t that they’re too far apart. It’s that they’re too close together. And the reason they’re too close together—musically, culturally, and socially—has little to do with their past and everything to do with their present. Traditional jazz and traditional bluegrass are both museum pieces; they might as well be embalmed. Their home is the festival circuit and the concert hall. Neither of these bands is about innovating or even preserving a tradition so much as they’re about ritually reenacting a moment when other musicians did one of those things.

Such reenactments don’t have to be aesthetic disasters. I have a ton of Del McCoury Band albums, and I love them, even though for the most part they’re not exactly what I’d call urgent. Traditional jazz at this late date is admittedly a harder sell. Jazz really is about looking forward, and there’s a kind of obscene emptiness in the reverent reanimation of its mummified remains. But even backward-looking jazz can be OK at times. I’ll still swear by Don Byron playing Raymond Scott.

So maybe this could have jelled somehow. Clearly the project was undertaken with an awareness of the problems besetting both bands. Juxtaposing bluegrass and jazz here is meant to be a way to shake off the embalming fluid. Put two dead traditions together and maybe, Igor, we will create life! And it doesn’t seem as crazy as Frankenstein’s plan, either. Why shouldn’t it work?

But it doesn’t. The end result of dead plus dead is, sadly, deader. “A Good Gal” highlights some of the problems. The PHJB’s mugging vocals would’ve been charmingly corny in 1930; in 2011 they make the teeth hurt. And the blues changes sound equally petrified whether played by bluegrass instruments or jazz ones.

“I’ll Fly Away,” though, is probably the most egregious failure. Right from the dramatic piano intro, the PHJB take is painfully smug, its cutesy retro swing an earnestly smiling parody of religious sentiment. The singing is unendurable—a paroxysm of pallid gospel overemoting that sounds strained rather than authoritative or heartfelt. It’s almost a physical relief when the vocalist shuts up and leaves the Del McCoury band to some sturdy quartet harmonizing. Inevitably, though, the PHJB comes back, and their dreadful singer finishes up triumphantly, having successfully made a travesty of two rich traditions.

I wish I could say that that the McCoury band’s harmonizing redeems the track. But I felt more like the track tarnished the harmonizing—or revealed its emptiness. There McCoury and his bandmates are, singing away in the middle of an irrelevant idiom . . . and you have to ask yourself the uncomfortable question, How is that different than what they usually do?

No doubt these wounds will fade and I’ll be able to listen to Del McCoury with pleasure again. At Saturday’s show in Naperville, they’re playing a set of their own (as is the PHBJ) before the groups take the stage together. But I’ll never be able to enjoy this album. When I picked it up I’d hoped for sparks or unexpected juxtapositions. Instead American Legacies delivers only easy truisms and bumbling failures. I wanted to hear two traditions stretch themselves, but the album just shows how thoroughly the elastic has gone out of them.