Cloning: A->B & B->A

Exactly. Back up each Mac to its own, dedicated TM backup drive, then restore each backup to the "other" Mac.

Each Mac should have its own backup anyway, but if you don't care about the contents of either one (some people don't) then you could get by with only one backup drive: Back up A, erase A, migrate B's content directly to A, erase B, then restore A's TM backup to B.

That is not ideal because it leaves each Mac without a backup in the interim.

As P. Phillips alluded to, Time Machine no longer backs up macOS itself. So the question of whether both Macs are running the same macOS version becomes critical.

Since Apple does not use that terminology an answer would require that you first define what you think it means.

To effectively create a duplicate of a Mac, create a Time Machine backup and then restore that backup to a different Mac:

Back up your files with Time Machine on Mac - Apple Support

Recover all your files from a Time Machine backup - Apple Support

Do both A and B run the same Version of macOS ?

Does A computer contain a Newer Version of macOS than Computer B does Not Qualify to run ?

Does Computer B contain an Older version of macOS that Computer A Does Not Qualify to run ?

Cloning software is available but be aware, starting in macOS 11 Big Sur some Cloning Developers even suggest against this as the ASR Process from Apple is perquisite to make it work as a Best Effort exercise

https://bombich.com/kb/ccc6/cloning-macos-system-volumes-apple-software-restore

Merci and thank you

Really hope OP takes this into account Before moving forwards

Welcome

To be fair about this, it was the work of @John Galt which was the tip off on this question