Q: iPhones have merged messages

Hello Daynafrommb,

Thanks for reaching out to the Apple Support Community. I know how important it is to keep your messages organized and separated, so I'll do all I can to help. It sounds like your devices are using handoff and continuity to forward messages. Have a look a this article, it will show you where to locate and turn off those settings: Use Continuity to connect your iPhone, iPad, iPod touch, and Mac

Specifically the sections on turning off the handoff feature and about SMS may come in handy. Handoff and continuity use bluetooth and Wi-Fi to sync some information between devices using the same Apple ID for iCloud. You may also have the text messages forwarding from one device to another which is addressed in the SMS section.

I also recommend to check in Settings > Messages > Send & Receive on both phones. Once in there make sure that only your phone numbers and email addresses are checked off on each phone. Having multiple phone numbers and each other's email addresses checked can cause iMessages to get sent to the both phones. 

If you have any questions or concerns, please let us know. The whole community is here to help.

-Best regards-