Rajnath and Sakshi are similar people I think. Both are power hungry and both are very sharp.
I think when it came to Rajnath-Raima, she played the role of the supportive girlfriend. She provided him with emotional support, complimented him on his smarts, helped him when he asked for her help, gave him real emotional sexual intimacy, etc. Basically, she pandered to him and he liked that. She probably also didn't mind doing that because she saw it as her love for him. That's why he really liked her. She made him feel good about himself and just good in generally and he probably did the same for her. It was mutually beneficial, the way society sees marriage as supposed to be working.
Whereas, with Rajnath-Sakshi, both are equals. Sakshi refuses to play the role of the supportive girlfriend. Instead of complimenting him, she makes fun of him. She doesn't necessarily provide emotional support as she believes in keeping her emotions at bay. Sakshi is an equal player and expects him to respect her. Heck, she was able to blackmail such a huge guy (powerful) to the extent where he had to marry her. So he probably grudgingly respects her, he has to. There's no doubt at all at Sakshi's intelligence. She's smart and gusty with good instincts. He may dislike her as a person, but it would be impossible to disregard her advice because she is smart. Heck she proved it by entrapping him. And Rajnath is very smart as well, he probably realizes that Sakshi is a master at playing games, thus her advice is useful. A smart person never disregards advice, no matter who gives it. Together, both are unstoppable. Devious, cunning, smart, they both have their own strengths and weaknesses. But together, their weaknesses disappear and their strengths gets stronger, hence why they've both managed to reach the top together.
And honestly, as much as Rajnath hates Sakshi and she hates him, both probably realize that without the other one, they'd be in deep danger. They're very similar and competitive and it causes friction in their relationship. But even this friction is overrided by their need to coordinate their intelligence to be at the top. I don't necessarily think their relationship is parasitic in nature. Neither one is living off the other beneficially. Rather, both are living off the other beneficially.
I think its more of a case of a general partnership attached with personal hatred. Without Sakshi's foresight, Rajnath would've never reached the position he has now. Without Rajnath's wealth, Sakshi would've never reached the position she has now. Its a difference thing that they hate each other, but it doesn't necessarily effect their partnership, especially because they have similar goals.
one another note, I'm curious to see who that person was who Sakshi genuinely loved before she married Rajnath. With Rajnath-Raima, it gives some insight into Rajnath as a person, aside from his evil side. With Sakshi, aside from her bond with her son, we haven't really seen her non-evil side. I want to see who she loved and why she did. I think it would definitely provide some insight into her character.
On another note, I pretty sure my views probably aren't 100% accurate because I've only watched this show on and off. If I'm wrong, I'd appreciate some discussion/ feedback!