A computer can only run one thing at any given time. But it can do many things inside a frame.
Maybe I don’t understand what your problem is, but it sounds to me like you simply want to call another function?
It is possible to run code in a separate “Worker”. See Web Workers, but they don’t really fill the same niche as co-routines in Unity.
@Owen_Attard is simple async method with callback is not what you need?