Using One Login on two separate WordPress installs

Hello,

First I would like to apolagize if this already have been posted
before somewhere. Just didn’t find it.

I have installed WordPress in the root directory and another
wordpress in the /sp directory. I need two separate ones for English and Spanish. I know that there are a lot of plugins for spanish but I need to have accurate custom spanish translation and separate posting.
What I need to have, is to have a SINGLE login for both and/or future
languages. Any Ideas? I would really appreciate an help. Thank you.