Cannot plug two Thunderbolt devices at the same time on MacBook Air M2

You can in fact put a Thunderbolt devices on both ports simultaneously. Yes, there may be one controller for both ports but that in no way limits the ability to have a Thunderbolt device on each port. Who ever gave you that impression is mistaken.

Here's an interesting article:

The Eclectic Light Company – Getting best performance from Thunderbolt on Apple silicon Macs: a practical guide

They say that in Intel Macs, the Intel Thunderbolt controllers usually support two ports per Thunderbolt bus, so the ports have to share bandwidth. (They do not say that you can only use one port of each pair to attach Thunderbolt devices.)

They say that in Apple Silicon Macs, there is often a dedicated Thunderbolt bus for each Thunderbolt port - giving the M1 Max Mac Studio as an example.

There's a lot more detail in the article, as well as tips on how to check the arrangement on your own Mac, and how to arrange Thunderbolt peripherals for best performance.

If there was only one controller for every two ports, are you sure that this would prohibit plugging things into both ports at the same time?

Instead of, say, possibly resulting in dynamic sharing of “a single port’s worth” of bandwidth between the two ports?

I do not know the arrangement on the M1/M2, but on several of the Intel-based MacBook Pros there was reportedly one controller for every two ports.

What does this mean? "After deep investigation in an Apple Store"

Did you consult at the Apple Genius Bar or just draw conclusions from what you observed? If you consulted, what did the Apple Genius say?

Although admittedly not an M2 machine, our two 2020 27" iMacs have:

Connections and Expansion

• 3.5 mm headphone jack
• SDXC card slot (UHS-II)
• Four USB-A ports
• Two Thunderbolt 3 (USB-C) ports 

I think it's a bit of a stretch to think the M2 chip Macs are less capable then the 2020 Intel models.

Each of our two 2020 iMacs have both TB3 ports is use successfully with a Samsung T7 SSDs.

But again, Apple has does make significant changes to ports.