They are at once the best of friends and the best of enemies. Where once they soldiered side by side for Clermont, this week the French club's head coach Vern Cotter and erstwhile assistant Joe Schmidt, whom he lost so reluctantly two seasons ago, plot against each other.
With respect to the other side of the draw, the prize of European glory awaits both men. So too the prospect of a rarely achieved domestic league and Heineken Cup double.
Little wonder that when coaching vacancies arise in their native New Zealand -- the latest a possible post at struggling Auckland Blues -- their names are automatically appended in news reports down south.
Both men, however, still have much to achieve in their respective posts. Already, they have exceeded expectations: Schmidt, in succeeding Michael Cheika's maiden European success with a thrilling one bearing his own enigmatic imprint; and Cotter, with Schmidt in tandem, in finally annexing the Top 14 title in the club's 98th year.
This, Clermont's centenary, has been mapped as the moment when they finally scale the European peak. Cotter's determination to outdo his comrade will add extra steel to an already mouth-watering clash.
Unlike its soccer equivalent, rugby's grand European prize admits an inherent sense of decency, far removed from the legitimate aggression tolerated beyond the white line.
Last week, both men spoke at length on the telephone. Each knows the other side's strengths and -- admittedly very few -- weaknesses intimately, so neither man feared betrayal. Instead, they exchanged familiar pleasantries, Schmidt bemoaning his limited time on the golf course, Cotter boasting of his latest hunting exploits deep in the mountainous Auvergne countryside.
Just as this is a meeting of superlative coaching minds, it is also a battle of European heavyweights. Clermont have this season recruited effortlessly to fill the gaps left last season.
David Skrela -- who is injured for this match -- Regan King, Sitiveni Sivivatu, Lee Byrne, Nathan Hines and Gerhard Vosloo replaced Mario Ledesma, Martin Scelzo, Sione Lauaki, Napolioni Nalaga and Benoit Baby.
Leinster, who desperately wanted to keep Hines, have added a World Cup winner in Brad Thorn, but their playing roster remains largely unchanged -- as it should, given their status as Europe's dominant side since 2009.
And yet it is difficult to understate the influence that Schmidt has left behind in France -- especially after, when already touted as Cheika's successor, he brought Clermont to Dublin for that unforgettable one-point defeat for his former side in 2010.
"He brought a lot to our club as he taught us the basics and brought in our attacking lines," says Aurelien Rougerie, who has always maintained that Schmidt and Leinster were a marriage made in heaven.
Morgan Parra is even more effusive.
"Joe taught us a rigorous approach, not the usual French way of doing things like we were used to," says the canny scrum-half, whose half-back partner will be Brock James -- he of the RDS meltdown two seasons ago.
"He brought a lot to our club with his personal touch and led us, along with Vern, to that long sought-after French championship title, so we owe him a lot.
"He will certainly use his insight and knowledge of our club to prepare his men. He knows us by heart -- and it might well give them an edge against us."
Clermont, a bit like Leinster, are regarded as a family club, despite their extraordinary playing budget, and children and wives are made to feel at home. South African World Cup-winning captain John Smit, who played under Schmidt and Cotter and whose current side, Saracens, were ejected by Clermont in the last round, still fondly recalls his one-year stint in northern France.
"Clermont have phenomenal players and have great financial backing. They invariably use their money well, buying just the right players who suit the club," says Smit.
However, as certain other French clubs have recently discovered -- be it Racing Metro's attempts to renew their halcyon days or the perennial shortcomings of Biarritz in Europe -- money isn't everything.
"They have an amazing structure from the academies up and an outstanding coaching staff -- they are fully aware how developing players will fit in," Smit adds."What is so special about Clermont is the magnificent relationship between players and the supporters, who pack the ground every weekend of a home match.
"From a player's point of view, that passion is a huge incentive to make every effort to help the team be successful."
Leinster second-row Damian Browne, having spent three seasons with Brive in the Top 14, is well placed to assess the undeniable threat Clermont pose to Leinster's ambitions.
"They have everything really," he says. "They're the kind of complete test for anyone. You know you're going to be taken on up front. They're going to be hugely physical. Then they have these backs who can play a special brand of open-running rugby. It's literally coming from every angle and they're obviously very good at it.
"They've definitely been quite outspoken about their focus on the Heineken Cup this season. It has been known to be a secondary thing to the Top 14 title. But they're obviously targeting it this year and they want to win it."
As well as the coaching links, a playing link endures through the prism of the irascible Hines.
"He is a great addition because he's a nightmare to play against," says Shane Horgan. "We knew that when he was playing for us. Apart from his skill set, his off-loading and carrying and line-out work, he's also a nightmare.
"He'll disrupt rucks, he'll stand in the wrong places, on the wrong side and pulling people around the place. All the things we were delighted he did for us.
"So he won't have changed much in 12 months. He's one of their key players. If we allow him to play as he can, Leinster will have huge difficulties."
The final texts between Schmidt and Cotter will have reflected their good wishes for the other. This week, they will studiously yet politely avoid contact until the inevitable pre-match greeting. Regardless of the result, their post-match embrace will be keenly felt.