Finding a Thunderbolt 2 to Thunderbolt 3 converter for external graphics card on MacBook Air 2017

Unlike the StarTech adapter, the Apple Thunderbolt 3-to-2 adapter is bidirectional. That means that you can use it to connect Thunderbolt 3 devices to Thunderbolt 1- and 2-equipped Macs.

However,

• Apple's Thunderbolt 3-to-2 adapter does not transmit power from one side to another. There would be no power to operate the bus-powered SonnetTech Thunderbolt to dual HDMI adapter.
• The SonnetTech Thunderbolt to dual HDMI adapter is not an eGPU. It depends upon hardware video output. The OP's Mac only supports one external display. So if you could connect the SonnetTech adapter to the OP's Mac, it would not be able to drive two displays in extended desktop mode.
• SonnetTech's Mac compatibility list does not include the OP's Mac. Only Macs with Thunderbolt 3, 4, and 5 ports which have hardware support for connecting two or more USB-C or Thunderbolt displays.

I suspect that the SonnetTech device to which you refer is not an external GPU, but a Thunderbolt adapter that uses hardware video output from the computer.

https://www.sonnettech.com/product/thunderbolt-to-dual-4k-hdmi-adapter/overview.html

Your 2017 MacBook Air only supports a single external display.

MacBook Air (13-inch, 2017) - Technical Specifications - Apple Support

Even if you could connect the SonnetTech adapter using an Apple Thunderbolt 3-to-2 adapter (which would appear to require some way of overcoming the issue of connecting two male plugs), that would not give you the second video signal needed to drive two displays.

Note that even with the aid of the Apple adapter, you cannot connect a Thunderbolt 4 or 5 dock to a Mac that has a Thunderbolt 1 or 2 port.

Other World Computing – The Simple Guide To Thunderbolt Forwards And Backwards Compatibility

"Thunderbolt 4 devices require a Mac or PC with a USB-C Thunderbolt port. That means that it’s possible to use Thunderbolt 4 devices with host devices that have Thunderbolt 3 ports, but using them with host devices that have Thunderbolt 1 and Thunderbolt 2 ports—even with an adapter—isn’t possible."

Thunderbolt3 to Thunderbolt2 adapter is available at online and retail AppleStore which is reverse compatible, so you can connect Thunderbolt 3 devices to your MacBook Air.

Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter - Apple

Mini DisplayPort is not the same thing as Thunderbolt 1 and 2. The Thunderbolt 1 and 2 ports on old Macs have Mini DisplayPort connectors, and can act either as Thunderbolt ports or Mini DisplayPort ports depending on what you plug in. But a regular Mini DisplayPort port would not support Thunderbolt.