Separate login for user / administrators using mongodb
You should not have two collections, that will give you headache. 

Have a user collection which has admin and regular users.  You can then query users as a whole or look for a given type only. So if you are looking for user id x you don't have two search two collections.

As for the question on duplicate ids with 2 collections, it simply depends how the ids are generated.  You say your ORM does it... so you will need to check the doc on how it does it.