Adaptor and Cables need for using iMac 27(2012) as a external monitor for iMac 27 Pro

Carefully note the macOS restrictions on Target Display Mode in the article kaz-k linked:

The HP is the far better option. TDM is dead tech and was a bit of a pain to set up. You may have fewer display options with the iMac than with the HP. When I tested TDM using a 2010 iMac 27 as the monitor for a 2012 Macbook Pro ( both were TDM-capable), I could only mirror the Macbook Pro display on the iMac, not extend it which was what I really needed.

Apple started the deprecation of TDM in late 2014 and threw the last shovel of dirt in its face five years ago.

Forget about using the 2012 iMac as a display for the 2017 iMac Pro.

What you need is a short high speed USB-C to HDMI cable to connect the HP HDMI display to the 2017 iMac Pro.

for example: https://www.amazon.com/dp/B08DNZJ4YC/ref=sspa_dk_detail_0?

The iMac Pro(2017) can run anything from macOS 10.13.2 (High Sierra) to macOS 15.* (Sequoia). If you hold it back to Catalina or earlier just so that you can use the 27" 2012 iMac as a monitor, that would be a bit of a waste.

It would be better to take the $80+ USD you would have spent on Thunderbolt connection hardware ($50 Apple Thunderbolt 3-to-2 adapter plus $30+ Thunderbolt 1/2 cable), and put that towards the purchase of a real 27" 2560x1440 (2.5K) or 3840x2160 (4K) monitor. You can get some 27" 4K monitors with IPS panels and 100% or near-100% coverage of sRGB for as little as $300 – $350 USD.

Hi,

Target Display Mode technology is little bit old technology, though following article could help your questions.

Use your iMac as a display with target display mode - Apple Support

I'm not sure your iMac Pro is capable to connect two monitors at the same time.

You're welcome.