Usually, an effect is the result of many causes, acting independently. But we simplify matters by attributing a single cause to an effect. The world is generally multivariate, or has multiple forces acting simultaneously.
Sometimes, two or more things have to happen simultaneously to produce an effect that we notice. As an example from films, Hindi films, the fact that Rajesh Khanna acted out songs sung by Kishore Kumar in a film called Aradhana (composer SD Burman) benefited both immensely, and each became a superstar in his domain. Both had some success individually, more so Kishore Kumar when he sang for Dev Anand earlier in the sixties, but the success multiplied manifold when he teamed up with Rajesh Khanna in a string of superhit songs. Dev Anand had equal success with songs sung by Mohammad Rafi and Kishore Kumar over his lifetime in films. Sometimes, like in Guide, both sang for him.
Rishi Kapoor initially had Shailendra Singh singing for him in his debut and a few subsequent films, but the songs he performed in films where Kishore Kumar sang for him (Khel Khel Mein, Hum Kisise Kam Nahi, Karz, were some) took him to dizzying heights of popularity. The music in this case was by RD Burman for the first two and Laxmikant Pyarelal for the third-they also played a role.
Just a couple of examples to show that many variables -acting together (interaction effect) or independently-may result in a particular effect.
