Basic file signing question

[Jack Kaye]

Greetings all,

I have what is probably a very basic question but for some reason I
can't seem
to find the answer anywhere online and was hoping one of you GnuPG
boffins could 
assist here.

I am trying to digitally sign a file as I encrypt it.

The encryption part is easy:

gpg -r <their key> -e <file>

Signing it is also easy:

gpg -r <their key> -se <file>

But when I sign it this way, it just seems to take the first key in my
keyring.
I have generated different public keys to send to different people and
would like 
to sign the encrypted file with a key of my choice.  Is that possible or
am I just 
not understanding the process of creating a digital signature?

Thanks in advance,

- Jack