I'm currently struggling with this issue. I have two sets of very similar DataObjects (share standard Title, Slug, Date, etc.). For the page they will be showing up on, I need to pull in the most current 2 items, regardless of which type they are. I've tried a couple different methods with little success. Even in cases where I have had success, the returned object did not work as intended in the template file.

Any suggestions would be appreciated.

It's not very clear what you are trying to achieve. Some code would help.

Sorry, should have included some code from the start. Here's a basic idea of what I'm working on:

$numPosts = 0;
$eduOffset = 0;
$flexOffset = 0;

$posts = array();

while ($numPosts < $limit){
	
	if($this->EducationPost()->exists()){
		$eduStream = $this->EducationPost()->sort('Created', 'desc')->offsetGet($eduOffset);
	}
	else
	{
		$eduStream = null;
	}
	if($this->FlexPost()->exists()){
		$flexStream = $this->FlexPost()->sort('Created', 'desc')->offsetGet($flexOffset);
	}
	else{
		$flexStream = null;
	}

	if((is_null($flexStream) && !is_null($eduStream)) || $eduStream->column('Created') > $flexStream->column('Created')){
		array_push($posts, $eduStream);
		$eduOffset++;
		$numPosts++;
	}
//Further checks like the one above

return $posts;

Essentially, I want to look at all the EducationPosts and FlexPosts associated with the page, then take the most recent $limit number of posts, type agnostic, and return them as an iterable list that I can loop through on the page.

The problems I'm running into are twofold. This implementation fails when I try to check which type of post is more recent (which I could mitigate by changing the way the checks work, but it feels like it'd be overcomplicating the issue) due to the ->column() call. Additionally, when testing just to make sure the returned object is iterable, the template does not iterate through the list of returned objects. the code for this snippet is as follows:

<ul>
<% loop ListPosts(3) %>
	<li>$Title</li>
<% end_loop %>
</ul>

I think you need to return the objects as an ArrayList() object in order for the template to be able to correctly loop over them. See https://docs.silverstripe.org/en/3.1/developer_guides/model/lists/#arraylist and http://api.silverstripe.org/3.1/class-ArrayList.html