To sum up the thought process.
The big ambiguity to building a double barrel is to have a the same time a strong assembly along with an easy procedures to deal with the sticky stuff between the ribs that does not involve extravagant temperatures because the hotter we go in flame, the harder it is to handle expansion and warping and elongation of the tubes during the... ahem... hard solder brazing process... no wait... the brazing rod soldering process... ooohh... how about plain soft solder soldering? Yeah... that's must be it.
Can we have it both ways? No. Well, yes, but not really.
If we could do without much of any barrel alignment and regulation, we'd have a homerun. Machine everything to tight spec, spot it all together and bam, ready. But nooooo... we want the lightweight and svelte and sexy side by side guns with the complicated three dimension alignment.
Now, if we could live with keeping our good eye in the same plane alignment as the over and under barrel assembly, then the job would be a snap. Stack the tubes on top of each other, glue, done. Strong hot temperature stuff will do, it's a quickie. (Of course, to accept the over and under, we have to settle on other flabby points - but that's not the topic here)
Still, we don't really want side by sides that are as mushy as wet noodles, so, we make the joining of the barrels a long and sultry operation and the expertise of the skilled people who do this make it last with the use of the somewhat low temperature and at the same time somewhat hard solder that does contain the good silver. But it is not technically soft solder, nor hard solder nor brazing rods?
I am probably way oversimplifying the whole thing, and I don't really remember the initial question. Pound us to the ground with details, I like it.
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Here it comes

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Here's a thought to solve all the problems in the world.
Our side by sides come with two barrels sleeping next to each other while we gaze straight over them, aligning our peeping eye with the rib and the dot sight. Hence the complicated triangulated triple beams that are hard to spot all into one hole at, say, 35 yards from the head. Kinky.
The over and under keep it straight in that the two barrels along with our eye-line hold themselves strictly into one vertical invisible sheet. Leaving only one tilting angle to adjust and play with between each. It's just up or down.
So now... are you still thinking about your gun?
On our side by side - instead of gluing the sighting dot on the middle of the top rib, like, at twelve o'clock at the end of the gun, why not push it halfway down to the side of the left barrel, back to nine PM - it would be like shooting an over and under in gangsta-style. On top of the coolness factor, we'd eliminate the regulation problem of the sxs, and it only seems to be a slight change of habit to get the eyes to follow. It even seems more ergonomic since we mostly shoot with our right eye and right shoulder so there would be no more twisting of the neck over the cheek piece.

I have a feeling I am going to miss a lot of birds on my next adventure.
Heck, to think of it, if dogs had driving licenses, they'd go about cruising down the highway with their head sticking way out of the left hand driver window. I like dogs, they're cool. We're on to something here, something our dogs know about and we don't. I'll bet their tails will stop wagging when they see what we're up to next.
Woooo hoooo