Is there a way of displaying an entry twice depending on postDate and a custom field date in one loop?
You could probably do this purely from Twig / a template, but I think it's going to end up being much easier from a plugin / PHP.

The general gist of it would be:


Grab all entries from section 1 ordered by postDate.
Grab all entries from section 2 ordered by releaseDate.
Create a new array and loop through 1 and 2 inserting via your custom sorting logic.
Return that new sorted array back to the template.