This month, Young and Simon both begin tours that will take them to 40-odd cities apiece: Simon with a 17-piece ensemble including three African guitarists, Young with his longtime collaborators Crazy Horse, the transcendentally crude three-piece group he calls “the world’s third best garage band.” Simon, who kicked off his tour in Tacoma, Wash., last week, drilled his group off and on since this summer to master the tricky songs from his 1990 album, “The Rhythm of the Saints,” which blend Brazilian drums, African guitar and Manhattanite lyrics. They also re-create South African-inflected material from “Graceland” and re-imagine Simon and Garfunkel favorites like “Cecilia,” now done in West African high-life style. Young, who opens in Minneapolis on Jan. 18, hasn’t gotten around to rehearsing yet. (He’ll put in a few days at Prince’s Paisley Park studio, but mostly for the folks doing sound and lights.) This may be for the best, since Crazy Horse is a band that shouldn’t think too much. As E. B. White warned James Thurber when he began shading and crosshatching his drawings: “If you ever got good, you’d be mediocre.”

The perfectionist Simon, 49, and the primitivist Young, 45, show just how far a couple of guys from similar backgrounds can diverge. Both were middle-class city kids: Simon’s father was a college English teacher in Queens, N.Y., Young’s a Toronto sportswriter. Both played rock and roll in their teens: Young’s band (like hundreds of others) was called the Squires; Simon and boyhood friend Art Garfunkel, calling themselves Tom and Jerry, had a song called “Hey Schoolgirl” on the national charts when they were still in high school. Both Young and Simon took up acoustic guitar and quasi-poetic songwriting under the influence of Bob Dylan. By the mid-’60s both were folk-rock stars: Simon and Garfunkel’s string of hits began with “The Sounds of Silence” in 1965; Young joined the influential band Buffalo Springfield in 1966. And both still love the dumb, innocent rock and roll they grew up on. Simon performed The Del Vikings’ 1957 doo-wop hit “Whispering Bells” with Ladysmith Black Mambazo on his “Graceland” tour and hopes to do the Dells’ 1956 “Oh, What a Night” with his new group; on “Ragged Glory,” Young covers Don & Dewey’s 1958 garage-band standard, “Farmer John.”

This isn’t mere nostalgia. Both Young and Simon still subscribe, more or less, to the ’60s’ essential article of faith: that the spontaneous is better than the calculated. That’s why kids from “good homes” took up both rural folk music and inner-city R&B in the first place - and why Simon, in creative crisis, recorded rhythm tracks with Third World musicians and used them to inspire new songs. “When the groove is there you have the heart and the body going,” he says. “When you start to add the brain” - he means chords, melodies and, finally, words - “the editing process tells you what’s right and wrong.” The African and Brazilian musicians Simon found weren’t unsophisticated - just fresh. “These guys are experienced studio players,” he says. “They can change it around, follow directions, transpose, do all that stuff. Very, very good at the record-making process.”

Simon should know: his own records have always been meticulously detailed, from the faintest finger-picked guitar part to the sequencing of the songs. (“The Rhythm of the Saints,” for instance, begins with a song in A-flat major, then goes to a song in A minor, then to one in B-flat major; these half steps up, Simon believes, give the spirit a corresponding lift.) Prompted by the rhythm tracks, the songs he wrote for “Graceland” and “Saints” may have welled up from Simon’s unconscious, but he gave them a good going-over. After decades of listening to music, he says, “your aural library becomes extensive enough that, you think, “That’s the way this thought should be expressed ‘It may not even be the kind of melody that I like, but I think, ‘Well, it works on this line’.” The words, too, have been worked and reworked to achieve an imagist precision: “I had a dream about us / In the bottles and bones of the night / I felt a pain in my shoulder blade / Like a pencil point? A love bite?”

Neil Young isn’t returning phone calls lately - even from his manager of 25 years. (Maybe eccentricity, maybe his demanding private life: he has two sons, born in 1972 and 1978, with cerebral palsy.) But one friend says he writes his songs in minutes. Sounds right: lines like “My heart goes running back to you” suggest Young hardly labors to find fresh ways to express familiar thoughts. The aptly titled “Ragged Glory,” though, makes it seem silly to apply such standards to him. All the words really need to do is sketch an attitude (usually angst), then get out of the way of the texture: his whining, tortured voice and burry, smeary lead guitar, cranked up to the edge of feedback. At the end of one song on “Ragged Glory,” he lets the amp growl and howl away for more than a minute.

In seeking energy and inspiration, Young has never needed to go farther afield than the American-Canadian heartland - and his own receding adolescence. Last fall he told Spin magazine that he wrote some of the songs on “Ragged Glory” out in his barn “in front of these fifteen old cars with their hoods up. Just me and all these old cars. And the spirits of the people who were in them.” To lighten the mood at one of the recording sessions at his California ranch, he sang standing in a bucket of cow manure. He records when the moon is full to “capture the magic”; during one session an earthquake shook the studio and nobody noticed.

On “The Rhythm of the Saints,” Paul Simon sings of “Spirit Voices” that “rule the night,” addresses the “Lord of the earthquake” and tells of walking “by moon” to drink a brujo’s potion. For Simon, this song is miles off his usual beat (self-doubt, tangled relationships), in a mystic Brazil yearned for (perhaps dreaded, too) by uptight gringos. “It will be all or it will be nothing. It depends on the passion / It will be dirty, it will be a dream.” For Neil Young, spirits, earthquakes and moonlight are business as usual - or so he’s doing his damnedest to convince himself. Neither Simon nor Young, probably, would walk across the street to see the other perform. No reason the rest of us shouldn’t check them both out. Among the other things they have in common, there’s nobody like them.