Can I edit Content Y within Content X Edit Form using Rules?
Entity reference and Inline entity form could help you.


Entity Reference:


  Provides a field type that can reference arbitrary entities:
  
  
  A generic entity reference field
  Four widgets:
  
  
  Two implemented on top of the Options module (Select and Radios/Checkboxes)
  Two autocomplete widgets (one with one text field per entry, one tag-style with comma-separated entries in the same text field)
  
  Two formatters:
  
  
  A "Label" formatter, with optional link that displays the label of the referenced entity (the node title, the user name, etc.)
  A "Rendered entity" formatter, that renders the references entity using a configurable view mode.
  
  Integration with Views (for both forward and backward references).
  Integration with Entity Metadata (and as a consequence with Search API and the like).
  Integration with Devel generate.
  Integration with Feeds and Migrate (>= 2.3) for painless data import.
  

Inline Entity Form:


  Provides a widget for inline management (creation, modification, removal) of referenced entities.
  The primary use case is the parent -> children one (product display -> products, order -> line items, etc.), where the child entities are never managed outside the parent form.
  Existing entities can also be referenced. Supports commerce_product_reference, commerce_line_item_reference and entity reference fields. Read the documentation here.