Salamander Archetype: Can they be "bridged" together?

Easily- unless you want the shelves to be the same level.
The easy way would be to join them with the left and right side shelves sharing the same two middle legs. These racks are nothing more than shelves with four holes for threaded rods with nuts to clamp and adjust the shelves. You could make your own for next to nothing, and using larger diameter rod and laminated butcher block type shelves have them be a lot better too.

Or maybe what you're talking about is two racks standing side by side and connected by just one shelf? Which would look kinda funny unless you're putting a TV or something in the space  between? But whatever yeah, you could definitely take something like a butcher block cutting board the size you want, drill holes in the corners, and use it to bridge the two racks together. Could probably do it even without having to buy any new hardware.

Yes, I am planning for a 3-1-3 shelf setup, with an amp stand on the floor in the middle.

I like your idea very much:
"...make your own for next to nothing, and using larger diameter rod and laminated butcher block type shelves..."

RODS:
Is there a vendor that you like? I took a quick look a few weeks ago, but did not find anything completely appropriate.

WOOD:
We have McBeaths (a wonderful hardwood dealer) in my city, and will look at them for the shelves.

Yes .... but....

All those nuts will drive you crazy. :)

What I mean is that you have to carefully assemble all the pieces in the right order. Nut, washer, rubber grommet, wood, rubber grommet, washer, nut......

I have a single stack rack for my bedroom and it’s hard as it is. Attempting to put together a bridge even more complicated. Maybe consider some racks from Butcher Block acoustics instead? Prettier, sturdier and easier to assemble.

BTW - So my own solution in my living room setup is to use two side by side racks and lay another shelf on top with nothing but gravity, and towels to hold it there.  TV goes on top of that middle rack.

The subwoofer and center channel rest between, the TV across. I’m quite smug about the setup, but in the future I want to buy fancier racks.

I like Butcher Block Acoustics very much. I will take another look and see if I can make that work.

Looking to place my Turntable, Full Size Phono Stage, and Power Supply on the left; Digital sources on the right; and Pre Amp and Amp in the Middle.

The amp is 17wx21dx11h (and has 10 tubes). I want the preamp on a shelf, well-above the amp.

OP
This is exactly what I did, built my own.
Not quite next to nothing but one heck o f a lot less than Salamander.

Home Depot supplied the stainless all thread for the rods and all the nuts and washers.
Amazon supplied 4 butcher blocks in Maple.
Drilled all the way through on three of them at corners and the fourth just drilled in approx 1 inch as this would be top shelf.

I love it!
Instantly adjustable shelves in a heartbeat for change of equipment.
Levels in seconds .
It's child's play to build it IMHO.

Check my system picture to see my rack.

Good luck.

Nice work, Uberwaltz!
https://systems.audiogon.com/systems/6466#&gid=1&pid=3

I will take another look at Home Depot's hardware supply capabilities.

What is the diameter and length of your rods?

Thanks Steve.
If I remember correctly the all thread rods are 36" long and 5/8" diameter.
Mine are stainless which cost a lot more than the plain galvanized but look a lot nicer and the nuts screw up and down on them a lot smoother than they did on the galvanized.
Yep I was playing with these rods and nuts in Home Depot quite a while before I decided which and what to buy.
5/8"  was thickest they had in stock and looking at it maybe 3/4" would have been even better.
However my rack is sturdy enough and as you can see I left room at the bottom for the amp which sits on another home made little stand.
Perfect?
Heck no.
Cost effective?
Hell yes.

Thanks for the tips, Uber.
This gives me a good direction.

https://www.grainger.com/ 
has a very good selection of threaded rods, nuts, coupling nuts and washers.
Including diameters up to 1" (and larger),
stainless steel, brass (and other materials),
and a variety of lengths.

You are most welcome.
Yes Grainger would likely have yielded more choice but I am not known for patience and once my mind is made up to do something I want it done yesterday... lol.
One big advantage of home brewed is that you can design it to be any way , shape or form to suit you. Within reason of course.
Good luck with the project.
Have fun and stay safe!