This is a how-to for newbies like me to set up a second linked iPhone (your wife's/husband's/partner's/kid's/mistress's). Some of the steps are redundant and may not be required but no there is no harm in double checking.
Follow these steps:
Double check wifi is enabled. No brainer.
Double check your wife/husband/partner/kid/mistress is set up with correct location(address) with the correct phone number on iPhone including Home, Mobile and iPhone numbers. Confirm with Siri by asking.
Enable Location Services for most gps apps you trust. As per some thread, this helps in geofencing. (How?)
Launch Map app on iPhone. It should bring up the correct location/address.
Install SmartThings app from App Store on the second device.
Register with the second email address (the person you sent the invitation to).
Open the ST App. It may say something in the lines of whether you want use this iPhone as a presence sensor. At this point the app may not respond to yes or done. Don't worry. Close the app and reopen.
Enable Location Services and Background Refresh for SmartThings app.
(Settings -> Privacy -> Location Services -> SmartThings -> Always)
(General -> Background App Refresh (On) -> SmartThings(On)
Double Check in Settings -
Notifications (Badges, Sounds, Banners)
Background App Refresh (On)
Use Cellular Data (On)
Within the SmartThings App:
Login with newly created linked account of course on the newly set up iPhone)
Add from Things -> Presence Sensors -> Mobile Phone.
Go to Home & Family. Click on the gear. Auto import existing presence devices. Select the second person. Set up push and text notifications.
Get out of the geofence and check if presence is correct and notifications sent.