The architecture
First you should think about the dependencies and hierachy of X to Y. And then orchestrate your nested components. Some examples:
Case 1: The hirachy is equal and they dont have dependencies, then create a root component for an independent navigation and on single load of the UI5 framework. Nest X and Y into this component. Might be good to have them as dependencies injected via a module loader.
Case 2: Y is part of X and is depends on it eg. control, models etc. Then X should nest component Y in order to forward these dependencies.
Case etc.
Implementaion
To load a new 'sap.ui.core.Component' in an existing one on a specific place eg. on a navigation route. You can simply add a component with ComponentContainer as XMLTag within the view on the desired place in the view structure to load the component there. Ps. make sure you provided the component namespace in your application bootsrap. The nested component should load and render itself.