I have created a simple Yes / No combo box that links to a cell in another
spreadsheet. However it is possible for people entering information in the
combo box to type something other than Yes / No.

How can I require that the information in the combo box is either Yes / No?

One way:
In an out of the way spot, call it cells A1 and A2, enter the word Yes
in A1 and No in A2. Create a named range called YesNo that refers to
A1:A2. In place of the combo box, highlight the cell and click Data
Validation; in the Allow box select List and in the Source box enter
=YesNo

This will limit user inputs to either Yes or No. Just to avoid future
gotchas if you use this again, if you include a blank space in your
range of cells, users will be able to enter anything in that cell.

Dave O
Eschew obfuscation