On iPhone, go to Settings
Tap Messages
Tap Send & Receive
Sign out of the Apple ID under iMessage if needed
Uncheck the phone number or email addresses you do not want used for iMessage
On iPad, go to Settings
Tap Messages
Tap Send & Receive
Sign out of the Apple ID under iMessage if needed
Uncheck the phone number or email addresses you do not want used for iMessage
On iPhone, go to Settings
Tap Messages
Turn off Text Message Forwarding if it is enabled
On iPad, go to Settings
Tap your Apple ID name
Tap iCloud
Turn off Messages in iCloud if enabled
On iPhone, go to Settings
Tap your Apple ID name
Tap iCloud
Turn off Messages in iCloud if enabled
On iPad, go to Settings
Tap FaceTime
Sign out if you want FaceTime separate as well
Use a different Apple ID on one device if you want complete separation
Delete existing message history on the device if desired