Multiple signatures on a single file

[Berend Tober]

Is it possible to have multiple persons sign a single file? If so, how 
is this done?

The particular scenario is currently this: Employees submit expense 
reports for business travel using a spread sheet. Current practise is 
the the employee fills out spread sheet via computer (or optionally 
prints blank spread sheet template and writes by hand with a pen), 
physically signs using pen and ink, physically delivers signed hardcopy 
to supervisor for supervisor pen-and-ink signature prior to payment 
processing.

Desired practise is to eliminate both producing hard copy and 
pen-and-ink signatures, and then re-work the process using gpg 
electronic signatures. Thus, employee would enter data into expense 
report spread sheet, save, gpg sign, mail to supervisor, supervisor 
would (presumably) open and review spread sheet, close without changing, 
gpg sign, and then return to employee or forward to accounting dept.

Sounds straightforward, but I didn't spot in the various 
manuals/guides/how-to's for gnupg how a second individual could add 
their signature after me.

-- BMT