Using maps on two devices

I doubt that you will be able to do this because Garmin's main maps are accompanied by smaller 'lock' files, these files give permission for maps to be loaded onto specific devices only. You can try to do this using Garmin Express and Garmin Basecamp, with Basecamp installed on a computer and your 50LMT connected to the computer , run Express and download City Street map to Basecamp using the 'Computer Only' option (see fig 3. in link Here), you will now be able 'see' the City Street map in Bascamp even when the 50LMT is not connected to the computer, from here you can use the Install Maps option in Basecamp with the 67i connected to the computer, Basecamp will 'see' the 67i and any maps you have on it, but the Install Maps option may only allow maps to be sent to the 50LMT and won't give permission to 67i

You can download free maps to Basecamp that you can install on your 67i using install Maps, see this link here :-
https://www.gps-forums.com/threads/can-i-get-free-maps.46545/