I wore a Milgauss on one wrist and an Apple Watch on the other for about several months before deciding it was never going to feel right. The watch I actually wanted to look at was on the wrong side and the Apple Watch ECG only works accurately in proper wrist contact, so wearing it face-down or loose defeats the purpose.
I ended up building a solution. A modular strap adapter that mounts to the mechanical watch via the standard spring bar system, with a built-in receiver that positions the Apple Watch on the same wrist, toward the forearm. Both have skin contact. Every sensor runs normally, ECG included.
The mechanical stays in its natural position. Nothing is modified permanently.One thing that matters a lot for comfort is lug width. The adapter works from 18mm to 24mm, which covers the Sub, the GMT, the Speedmaster, the Black Bay, most Seiko divers. The Apple Watch uses a proprietary connector so a short adapter bridges it to standard 20mm.
After two years of daily wear it genuinely stops registering. Day three felt normal. Week one it felt intentional.The product is called Smartlet. Awarded at Concours Lepine 2025, shown at CES 2026. Technical details at smartlet.io. Happy to answer questions on specific references or sensor behavior.