

I don’t know, do they get along at all? What do you mean by “getting along” with each other? Have you got any examples?
Psychos often prefer to do their things alone, although there are some exceptions, some even famous, like the Barbie and Ken killers whose real names escape me right now.
But do these people really “get along”? I don’t think so. When you put someone toxic with a regular person, you get a toxic relationship. And when you put two toxics together, they don’t cancel each other out. They’re still toxic. They’re still fucked up.
Just go and read/ask/listen to anyone who lived through a toxic relationship. Or an abusive one. In many cases the abuse goes both ways and this is exactly the type you’re asking about. It’s way more mundane than what you think.
You seem to be framing relationships in a materially transactional way, which isn’t how humans typically operate. That exists (ie. sugarbabes & daddies, or some arranged marriages in very religious cultures), but usually what holds a relationship together is the emotional and psychological exchange between the two people.
In cases where abuse goes both ways what holds them together isn’t that A is rich and B gives sex, but rather A uses their wealth to degrade B and B in turn gets revenge making A feel inferior in bed for example. Note it’s the emotional hook and not the action that matters. And it’s never just one thing, but lots of layers. So on top of that maybe B is also manipulative in a specific manner which makes A want to lash out in some specific passive aggressive way and so on. They get into a loop that gets more and more complex as the relationship progresses. And neither party wants to break up, even though threatening to do so might very well be part of the equation, because they’re hooked up on getting the next upper hand.