I have built a lot of mantels over the years, some with vertical surrounds, some just floating, ornate with crown moulding and others that are just barn beams, but I have never done one that was double sided.
I have seen fireplaces like that, usually gas units in retail locations or restaurants.
This one goes to the car dealership that I have been working on for the last few weeks. Why the veneer? I don't know. The rest of the job has been a handful of laminate (HPL) colors.
The main boxes are just a shell to cover existing support blocking. It's just 3/4" plywood halves that are aligned with Dominos and snap together with some Lamello Clamex connectors. The veneer is some really good looking rift white ash. Sadly though, it is going to get stained to match one of the laminate colors.
You would be forgiven if you missed the seam. It happened to someone at the shop today. I was asked if I veneered over the joint, so it couldn't come apart. I knocked it apart to show him....stunned. I just hope it remains that good after it gets stained. The edges are "end grain" so they may darken more?
I just wish that the veneer would have been long enough to wrap around the corners, but it was only 8' sheets.

That is an amazingly well done join!

Looks like you're using Kreg joints? Any reason for this over the Dominos, for extra strength maybe?

Amazingly good fitting butt joint!

Just Domino’s and the Tenso connector hold it in alignment and together?

Yes, just 6mm Dominos and one Tenso per side. This should be the easiest install ever. It is sized to just snap over an existing support.

That is an amazingly well done join!

Looks like you're using Kreg joints? Any reason for this over the Dominos, for extra strength maybe?

I have never even seen the Kreg joints? didn't know there was such a thing. The thing I like about the Lamello Tenso is that they are totally tool-less to assemble and nothing shows afterword. The are not "permanent" though. The will snap back apart, if you can create enough force in the right direction.
This is the point of the Dominos. Once those are glued in, it will take major violence and destruction to get it apart.
 

That seam is perfect. Even knowing where it is and zooming, it was difficult to find. Excellent work

Thank you. That means a lot coming from you. I have seen the impressive stuff you have done.

Are they not Kreg type screw fasteners? Or are they for the Lamello fasteners?

@luvmytoolz  yes those are Kreg pockets, but that is just part of the glued sections. Those will not come apart again.
I thought you were referring to something that I had missed out on somehow?

@squall_line I am a bit mixed on the castle pockets. They work, don't get me wrong, the company has had one for years. I find that there are some problems with them though. Since the cutters for the pocket and drilling are separate, there can be alignment issues. The main problem though is that they are effectively exclusive to 3/4" material. You can insert thicker material, but you can't move the pocket to center it. (at least on the older, floor standing model we have) "Thicker" is also limited to about 1", because of the clamping mechanism. There is also the lack of portability. Order of operations comes in there. If you need to add a pocket somewhere in an assembly, you're out of luck.
The advantages of the drill-bit style (Kreg or others) are exactly the opposite of that. You can go thinner and quite a bit thicker (centered). The alignment is never a thing, as the holes are simultaneous. Plus, there are many ways to use the same drill bit to get the same results. We have a full-sized floor model that is 100% air-driven, no electricity required, except to run the compressor of course. Then there are a lot of different hand-held or bench mounted alternatives.
For me though, the biggest advantage of the drill-bit style, is the plugs. Often this doesn't matter, because the pockets are inner structure fasteners (never seen). Once in a while, like in this case, the pockets are on the surface somewhere. They will not be seen, because they are veneered or laminated over, but I prefer to fill the holes to keep them from being punctured or dented later. It's not hugely important on vertical surfaces or under the thicker laminates. On horizontal surfaces with veneer or the thin (vertical grade) laminates, it's just a safer bet to fill them.
You can fill Castle pockets, but it takes much more fiddling.