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.