I've never thought to do this with WordPress until now, but I was wondering is it possible for two templates to be chosen to create a post or page in WordPress?
When you are on the 'Edit Page' there is the 'Page Attributes>Template' section. When a custom template is chosen that will generate the main layout of the page which includes the region for an
iframe comes into play when the
custom-field below the content editor is filled out, which would automatically call the second custom template when the page is published.
So basically, for the sake of a visual, imagine a comparison site with side-by-side view of two pages. Only now think of that example as a side-by-side view of data inputted into
the_content (WP content editor) and
custom_field (custom field box/field below the WP content editor).
iframe is needed, but if you know what other code could be used to produce the same results would be fine.