I'm trying to create two content types at once in one form. One is a parent page the other a child page
The Inline entity form module was built for pretty much that exact use case:


  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.