Using same users over multiple CMSs

[Connie]

There are many projects out there to simplify logins and use only one registration, but GetSimple does not
and there are many reasons for that

This is GetSimple and not GetEverything ;=)

you could try to write a plugin to read from a database, but still you have to activate this user in Getsimple.
Don't make it too complicated. Why not use the same username and password in the systems which you want to integrate, regardless that you must define the users differently in the systems?

This is administration. Normal job.

Just train your memory and it will be ok ;=)