They are the closest of rivals and the best of enemies - but on the final day of the regular Pro12 season Scarlets and Ospreys need each other's help.
Fifth-placed Scarlets are trying to overtake Ulster in fourth and claim a place in the end of season play-offs.
Ulster are at Ospreys - who need to overtake Munster to snatch sixth-place and Champions Cup qualification.
Scarlets play Munster in Thomond Park where a win will do themselves and Ospreys a favour.
With so much at stake and their results likely to have a major bearing on the other Welsh region's hopes, there's more than usual interest in the rivals' matches.
Here's a rough guide to the issues and permutations.
How they stand
What Ospreys need Scarlets to do
Beat Munster, and stop them getting two bonus points.
In that scenario, a win for the Ospreys will clinch sixth place and the coveted spot in the European Champions' Cup next season.
If Munster manage a losing bonus point, Ospreys will need to win with a bonus point against Ulster.
A losing bonus point and four tries for Munster and Ospreys will be facing their first season as spectators to European rugby's top tournament.
What Scarlets need Ospreys to do
Stop Ulster matching what whatever Scarlets do in Munster.
Scarlets need to get one more point from this weekend's matches than their Irish rivals, and an Ospreys win would go a long way fulfilling their side of this unwritten bargain.
An Ulster win would mean Scarlets needing a bonus point win in Munster - a tall order by any measure.
If Ulster win with a bonus point, then Scarlets' season is over - and Ospreys will be reflecting on a season of under-achievement compared with the standards they have set in the past.
What the players say
Ospreys forward James King: "Obviously we'll be supporting the Scarlets. We need to go for five points and hope that Scarlets don't let Munster have anything really, but that's a big ask at Thomond Park.
"I might send a text or two later to ask them to see if they can do is a favour."
Scarlets back Gareth Owen: "It would be nice for them to do us a favour wouldn't it?
"I don't know if they think they owe us a favour or not, but the Ospreys have something to play for.
"I haven't spoken to any of the players yet - they'd probably tell me where to go, I guess."
What the coaches say
Ospreys backs coach Gruff Rees: "Yes, they would be doing us a favour if they beat Munster and vice versa, ironically enough.
"We have to have one eye on Thomond Park - if Scarlets can pick up that extra point on them regardless of winning, then Ulster may have to chase some tries as well so the dynamic may have to change during the game.
Scarlets head coach Wayne Pivac: "I'll have a laptop set-up next to me with all scores on it - they'll be streamed as it's happening so we'll know exactly what the equation is so if we need to change our tactics we'll have the ability during the game."