Starting with two cubes, side by side.

Overlapped:

Rendered with SuperFly (FireFly doesn't do this, but I want to use SuperFly):

There are other objects that cancel each other when overlapping, but this is the one that's driving me crazy right now. Trying to create wall primitives, and want to overlap the patterns (brick) to line them up properly. It isn't really a cancellation, in that you can't see through the black space:

Doesn't seem to matter what materials are used.

Any clues, tips, suggestions?

Further experimenting indicates that it's the overlapping surfaces that are the problem. If you move one cube a skinch down and back, it sort of works. Anyway, it's not so much a cancellation as it is that the overlapped surfaces render as black.

Try subdividing the cubes to 2 and rerender.

Superfly render

                                        

Firefly Render:

         it is apparent that subdivision does not work in Firefly  

AWESOME!!! Thank you so much.

I still had some "noise" using subdivision = 2, but it worked perfectly at 4. Very, very, nice.

The subdivision will render longer.  Increasing the pixel number will get rid of the noise.

Hmm. How do I increase the pixel number? Is that one of the renderer setting?

Ah. Is this the setting you mean?

The "Render" setting?
This combination seemed to work well for me

That would be in your render settings in Superfly.

This is what I see on my system:

These are my normal settings. Do I have the setting you're referring to? Pixel samples? Although I do have that already cranked up a bit, so I'm guessing that isn't it. Sigh. I do hope this isn't somehow because my video card doesn't support something.

Another questiion: When I set the subdivisions to 2, the edges and corners of the cubes become rounded. Is there someway to keep them crisp?

Not much you can do with rounded edges from subdivision on a single plane mesh.  You will get distortions to varying degrees.  You would need to create the object in a modeling program where you can physically determine the amount of subdivision.  Poser's subdivision is more adaptive to organic models like figures.

It's a "feature".

https://projects.blender.org/blender/blender/issues/96475

Yet another example of programmer arrogance.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)

Can someone recommend an adequate modeling program?

Sometime long ago, like maybe 20 years ago, I built some articulated props which I cannot, for the life of me, remember how I created. They're complex enough that I don't think I built them in Poser. This is one, fully articulated. The chain bends, the lock unlocks, the cuffs open.

Somehow I don't think I built it in Poser (4, btw). I have this vague thought that maybe it was Dazd (Daz3d?) but I really don't know. Effectively, after a 20 year haitus, I'm pretty much a newbie again. So any suggestions about what app to use would be greatly appreciated.

A large portion of that could be built in Poser today. I like Blender for my modeling. With the new interface, it's a lot easier to use.

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I use Poser 13 and win 10

Hmm. Thanks for the tips. I am finding, with a little exploration, that I do seem to be able to do a lot more in Poser than I remember, so that's good. And I will go google Blender and read up on it. Thanks again!