Is the form sumitted within a network? Does the network use Active
Directory for user names and permissions? Does each user have a
specific logon to the computer or does everybody use the same login?

For starters, you'll need to pull in the managers login from the
computer. Then you'll want to match that with the manager field. If
that validates, then you'll allow the approver field to be shown with
conditional formatting.

You can also set a rule, that approver can not be the person who
originally submitted the form. This would eliminate the original signer
from signing the document. You also stated that its emailed to manager.
If thats all automatic, then the manager would be the only person
looking at the form anyway.

Josh