They are though. They're antonyms. There's basically no such thing as something that works as complete "only one of these things can exist at a time". But pleasure and pain are the closest I could think of for the Warhammer gods.
Plus, just beacuse you enjoy soemthing that is painful doesn't mean that pain and pleasure aren't opposite sensations. It just means you enjoy things that aren't good for you.
Going "Yes they are" is not an argument. And while completly either/or concepts are really hard to find, because of the infinite variety of circumstances, there's sure as hell better examples than pain and pleasure. A god of the rich and poor, or a god of war and peace.
On what basis do you assert that they're opposite sensations? They have completly different biological purposes; one is a signal that you're doing something good, and the other is a signal that you've either taking damage or are doing something that could do damage if pushed too far. But something can both be good for you, and be close enough to damaging to ping that sense. And there's plenty of examples of them existing in parallel IRL (see prior post), where it's not something that's not good for you.
Another timeline? You're saying this stuff doesn't already happen in the realm of Slaanesh?
In this timeline, Slaanesh springs from the anxities and hangups about sex in 80s Britain (though the transphobia is alive and well). So whatever happens in Slaanesh's realm is stuff they'd be totally appalled by, and also not at all even a little interested, no really.
Other timeline Slaanesh is probably a pretty cool dude/dudette. A bit much, maybe, but mostly ok. Probably not even a chaos god. They got replaced by... probably a god of tyranny (picking up on the feelings that led to Brexit, and also the economic situation of the not-old generation) or of bad weather (based on climate change)? I dunno, I'm not British, don't know what they're currently worried about. Probably still ambigious gender, because that's a vein of nastiness that's actually gotten worse in recent years.