two issues here:
a) pc credentials for two users {admin, limited}
b) access to the ISP pop3/smtp account(s)
on (b), you can configure any/all users to access the same pop3/smtp accounts
on your ISP, but suggest you be careful -- see next cmnt,
on a), different profiles are used for each user which will create a different set
of files on your PC, eg
\Docs & Settings\$limited-user$\Application Data\Thunderbird\Profiles\xx.default\Mail
which will be unique from the $admin-user$
if you carelessly use both logins to access the same postoffice account,
your email will end up spread across both accounts.
HOWEVER, if logged in as ADMIN, right-click a short-cut to TB, you can use
the RUN AS interface to launch TB as the $limited-user$ and thus keep all
the email in one location
the config for the $limited-user$ email is to configure it exactly as you did for the ADMIN,
(name/passwd, and the same pop3/smtp.domain.names)
you've started from the ADMIN perspective, so the files on your PC are relative to that profile.
1) copy the Profile directory from the ADMIN account to the same location
for the $limited-user$
2) reboot into SAFE MODE, take ownership of the
..\$limited-user$\Application Data\Thunderbird\Profiles directory
3) set perms for full access on that dir also
now the $limited-user$ will have the abilty to read/write to your postoffice,
and as long as ADMIN does the RUN AS trick, all the email will be owned by
$limited-user$