And the first came in as dramatic a way as possible
When the Frenchman blew apart the field in the 400m medley earlier in the week, he led from start to finish, a dominant crushing of his opponents to send the euphoric crowd into new levels of ecstasy rarely seen in swimming.
He was the overwhelming favourite, and he delivered.
In the 200m butterfly, he was not.
The baby-faced star was very much the underdog to Hungary's world record holder Kristóf Milák, albeit the second-fastest qualifier from Tuesday night's semifinals.
And, for 150m of this race, he looked it, trailing by 0.72 seconds and seemingly out of contention.
But then it happened.
The crowd realised instantly, rising organically to their feet as one, raising their voices loud as Marchand put in a remarkable burst to erase that hard-earned lead in the space of five-short metres.
The underwater work was perfection, Marchand coming up level with the Hungarian on the final stretch.
The tricolores were not waving, but being shaken in fits of angst, or else being frantically grasped like oversized, multicoloured nylon prayer beads.
The 15,000-strong crowd was screaming themselves hoarse as Marchand pulled level, then edged ahead, the crescendo continuing to rise until he touched first in the third-fastest time ever.
One has to feel for Milák.
Butterfly does not lend itself to an awareness of where your opponent is, but he would have known — 15,000 screaming French fans made sure of that.
While Marchand wears the weight of the crowd and nation's expectations as comfortably as a tailor-made suit, the crushing pressure seemed to transfer itself readily onto the 24-year-old's back as he was overhauled.
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Milák's final 50m was swum in 30.23 seconds, 1.57 seconds slower than his previous length and just fifth fastest of the eight finalists.
Marchand though, swam that last 50m in just 28.97 seconds, 0.66 seconds faster than anyone else managed during their final charge.
It was, quite simply, astonishing.
Once again, the outpouring of joy at the medal ceremony was something to behold, chants of "Léon! Léon!" pulsating through La Défense, the triumphant rendition of La Marseillaise stirring for the soul.
After the energy-sapping emotion of the comeback, and the equally draining medal ceremony, Marchand was permitted to skip the victorious circumvention of the pool to accept the adoration of the crowd at close quarters.
Because he wasn't done yet.
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Just under two hours after diving in for his butterfly race, Marchand was back, swimming the 200m breaststroke.
"Allez Léon! Allez Léon!" was the chant as Marchant marched out, into lane four alongside Australia's Zac Stubblety-Cook in lane five.
Nobody had ever won the breaststroke and butterfly medals in the same Olympics.
But there's never been anybody like Léon.
In contrast to the butterfly, this time Marchand took the race out from the front, first through 50m, under world record pace at the 100m, and the 150m too.
The crowd continued its rhythmic roaring every time their hero's head breached the water, a thunderous wall of sound forcing him forward with every stoke.
Stubblety-Cook has made fightbacks something of a feature of his own of late.
The former world record holder turned fifth after the first 50m, was third by the midway point, and then the chase was on through the second half of the race.
But in truth, this time Marchand never looked like being beaten, an almost regulation victory that was still cheered like another great comeback by the thoroughly sated home support.
That simply wouldn't have been in the script, even if that most would have thrown out a manuscript so utterly ludicrous as this.
Winning two gold medals in the space of two hours, at a home Olympics, in two disparate events, with the weight of expectations hanging over him, was simply extraordinary.
To put it into perspective, the last swimmer to win two individual gold medals in one day was Kornelia Ender in 1976 — and that was with the significant support from the nefarious laboratories of East Germany.
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The vigour with which La Marseillaise rang out across the arena this time was just as loud, with just as much pride as at any time during the entire history of the Republic.
Marchant was always destined to be a swimmer, the son of 1992 Olympian Céline Bonnet and two-time Olympian Xavier Marchand, both of whom were medley swimmers, with Xavier winning silver at the 1998 World Championships in Perth over 200m.
But it would have been a brave fortune teller indeed to suggest anything like this would have been remotely possible.
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