I feel like the core question/goal/ideal is to have two sim chips linked to the same number, so that requests (such as phone calls/texts) are cloned and sent to both devices.
I'd think that'd require some reconfiguration on their backend service though. I'm not too familiar with how telecoms work.
Linking to the same number happens at the iCloud level. Apple already has hooks in place with carriers to enable "Wi-Fi Calling on iCloud-connected devices", so an iPad/Mac can make/answer calls from the cellular number of an iCloud-linked iPhone (without going through the iPhone at all). The LTE Apple Watch is just another device like that.
I'd think that'd require some reconfiguration on their backend service though. I'm not too familiar with how telecoms work.