Can a witness server be a witness for more than one pair of mirrored servers? In the interest of saving time and money (licenses).

For example:

We have a mirrored SharePoint backend as well as a mirrored Lync SQL backend.

I want to use the witness for the Lync servers to be the witness for the SharePoint mirrored pair at the same time.
There's no reason why it can't be done. In fact, Microsoft documentation states that one server can be a witness to multiple sessions.