David Humphreys and Alan Solomons go back a long time together - but on Friday night in south-west London they will be fierce rivals.
Neither man would probably have imagined such a scenario when they were guiding Ulster to domestic success during the early 2000s.
As captain and record points-scorer, Humphreys was Ulster's fly-half talisman. combining his outstanding goalkicking with a tactical presence that made the province tough nuts to crack, especially during South African Solomons' coaching stint in Belfast between 2001 and 2004.
Ulster won the Celtic Cup under Solomons and they were also runners-up in the then Celtic League, while he oversaw a three-year unbeaten home Heineken Cup record. During that period, Ulster rose in the European rankings from 37th to 10th.
After Humphreys retired from playing in 2008 - he also won 72 caps for Ireland - he became Ulster's director of operations, with Solomons having a brief spell at Northampton before heading home to take charge of Port Elizabeth-based Eastern Province Kings.
Since 2013, though, he has been head coach of Edinburgh, and Humphreys now runs the show at Aviva Premiership club Gloucester, therefore adding an intriguing sub-plot to the European Challenge Cup final between the teams at Twickenham Stoop.
"I remember the first day Alan arrived at Ulster," Humphreys said.
"He sat the senior players down, explained what he wanted to do and told us things were going to change, and they did for the better.
"He has done something very similar at Edinburgh. He is an outstanding coach, he has been very successful in terms of taking teams and making them very successful.
"Edinburgh have always played some really good rugby and been a difficult team to play against, but what he has done is change the way they play.
"They have a strong set-piece, a strong defence, a very good kicking game and they are more conservative compared to Edinburgh teams of the past.
"The biggest reflection is where he has taken them in terms of the league table in the PRO12. Alan is very clear on what he wants. I'm pretty sure he has adopted the same mind-set with Edinburgh.
"We know we will play against a highly-motivated and organised Edinburgh side which will require us to play at our very best to be successful."
"I remember the first day Alan arrived at Ulster" - David Humphreys
Gloucester's, conquerors of Connacht in the quarter-final, bid for European glory, though, will be assisted by the presence of Scotland captain Greig Laidlaw in their starting line-up.
Laidlaw moved to Kingsholm last summer after spending seven years with Edinburgh, during which time he made close to 150 appearances and established himself as one of the northern hemisphere's leading scrum-halves.
Humphreys added: "Edinburgh have had quite a big change in personnel over the last couple of seasons with Alan's arrival, but the advantage we've had is Greig Laidlaw and his in-depth knowledge of how they play.
"We know where their strengths are, and we know their weaknesses.
"Sam Hidalgo-Clyne (Edinburgh's highly-rated number nine and Laidlaw's opposite number) is one of the future superstars of the game.
"We had heard the name but he is not someone I knew before this year, but when you look at his performances he has been outstanding.
"But if we go out and perform to our very best, if we produce a performance at the same level as we did against (Challenge Cup semi-final opponents) Exeter, it will take a very good performance from Edinburgh to beat us."