Even with single angle characters, its fairly easy to create the these characters talking to each other. Just gotta get creative either with the editing or the layout or both. Here's a couple of ideas...
One way is to draw your background in a way that you can pan the camera left and right and have it look like 180 camera turn. This is done by drawing the background warped in the middle.
Another is you can simulate over the shoulder shots with the front character either being in silhouette, or off camera enough so as not to reveal that you're actually seeing the front but rather give the illusion of the other characters back.
I don't have any of those character except for Rosenberg so ill post a couple gifs using him ...

Enough of the character is off camera that we cant actually tell its the front...i looks like the back.

Using a silhouette allows us to show more of the character outline but we need to darken so as not to reveal the actual angle. (i duid this one by exporting the chracter to photoshop, adding black overlay, and 10% Gaussian blurr. Re-imported the png back into CTA to use as a stand in.

Panning left to right, or just cutting from left to right will also give you the illusion in editing. If you warp the background so it conforms itself to the 2d camera movement it gives it better effect.

combine with silhouette for even bigger impact...
There so many ways you can pull it off, just be creative.

Ibis Fernandez | (available for hire)
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Professional Animator, Filmmaker | Creator of the highest quality (modular) G2 rigs for cartoon animator and developer of Toon Titan and Puppet Producer
Author of Flash Animation and Cartooning: A Creative Guide
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