Replace your strap
The watch adapters seat in your mechanical watch's lug holes in place of the spring bars. Classic, Shadow or Titanium. The spring bar tool is in the box.
The Apple Watch carries the screen, the notifications and the apps, and it charges every day. The Panther Eclipse answers the other side, screen-less stainless steel, no subscription, 14 to 15 days of battery, reading HR, HRV, SpO2, temperature and blood pressure on the inner wrist.
Smartlet keeps both on one strap rather than two bands.
The watch adapters seat in your mechanical watch's lug holes in place of the spring bars. Classic, Shadow or Titanium. The spring bar tool is in the box.

Your smartwatch attaches to the central part of the bracelet with its own adapter (Apple Watch, Whoop, Fitbit, Galaxy or Pixel Watch) or a pair of standard watch adapters. Zero modification to the mechanical watch.
Both watches sit on the same wrist. The smartwatch sits underneath, against the skin; readings taken there are not certified by its manufacturer.