From what I understand the users must exist in both databases. I have a similar problem on FMPS Advanced and the user authentication isn't working between databased and subsequently the "modified by" does not work at all.
As stand-alone databases it works great, but once it's on the server the method stops working.
The Account Name will be the account that was used to open what ever database your AE fields are in. You should be careful because it sounds to me like your second database is being opened using the default Admin account automatically. If you're at all concerned about security, this isn't a good thing. Even if the file isn't set to be visible on the network through FM networking, if someone knows the file's name they can open it by entering the file name manually. If they open it this way they will have administrative rights. What's more, if you haven't disabled the admin account in your other file, they may be able to access that as well.
You should have the same accounts/passwords in both files and don't set a default account/password unless you really want someone to be able to open the file without a password. If you have the same account and password in both files, the second will be opened using the account information entered by the user to open the first file. This will happen automatically; they won't need to enter the account/password a second time.
Hope this helps.
Keep in mind, too, that Account Name and User Name are not the same. To autoenter the user you need to set the field to autoenter by calculation, in which case you could create a calc that also inputs date, time etc as well. Modifications can also be recorded this way.
Here's a method which works for my employer:
We created a FileMaker file Login.fp7; each record contains the following fields:
User_Login - text example - "John Doe" or "John Smith"
gCurrentUserLoggedIn - global text field - is set to the value of User_Login at Login
True1 - always 1
Once Logged in; if you have a field True1 which is always 1 in your second database, you may setup
a relationship there File2:True1 = Login::True1 to fetch the value in the global text field gCurrentUserLoggedIn.
createby and modifyby may be setup to enter the value gCurrentUserLoggedIn. This works because a global
field value is local for each user.