I'm not having much success converting 3D clips into DCP. I have learned that some clips are simply 2D versions of 3D as you mentioned below and don't work. I have found some 3D clips that are 3D and have two panes of view, Left eye and Right eye. I have not successfully converted one of these yet. The file format is .m2ts like this one, https://www.demolandia.net/downloads.php?id=67931465

Once I was able to successfully complete a conversion without errors, but when I played it back from the DCP format, it played with two separate panes showing, left & right eye. What am I not doing, or not understanding with these 3D trailers?

That appears to be a side-by-side 3D clip - left and right in one image. I assume you made yourself familiar with these types? You have to configure the proper content 3D type so DCO-o-matic knows where to look for the left and right side images in your content.